Bluffing the Advance Scout

Earth is about to be invaded by a horde of hostile aliens. They outnumber us, outgun us, and have massively more advanced technology. We're doomed.

Except that they don't know much about Earth, so they've sent some advance scouts to find out how dangerous we are. If we can just persuade the advance scouts that we're too nasty to mess with, the invasion will be called off and we'll be left alone.

It used to be a common trope in the Golden Age of Science Fiction, but not so much these days. Even then, it was frequently Played for Laughs.

Comic Books

 * In Fantastic Four #2, the first appearance of the Skrulls, the FF bluff them into thinking that Earth is crawling with giant monsters by showing them pages from a comic book, pretending they're real photographs.
 * In one issue of the West Coast Avengers, the aliens test the WCA line-up individually, putting their battle robot (and ship's power systems) under increasing stress. Then they finally get to Moon Knight, who is empowered by the ancient Egyptian moon god. And they're testing the heroes in a dimension filled with moons.
 * In one issue of Exiles, the omniscient whatever-it-is that commands the Exiles sets up a chain of events that ends with some minor supervillain, feeling unappreciated, setting off a weapon that fills the Earth's atmosphere with foul-smelling gases for 72 hours. All because the omniscient whatever-it-is has foreseen that during those 72 hours an alien invasion fleet will arrive, scan the planet, and decide to move on to somewhere with a nicer atmosphere.
 * A fairly recent Mickey Mouse story had the mouse and his sidekick, Goofy, visiting an archaeologist-friend at Abu Simpel in Egypt, when they're abducted by Ancient Astronauts - specifically, the old egyptian gods and pharaohs, who left Earth back in the day after being insulted (and nearly accidentally fed to the crocodiles) by one of Goofy's ancestors. Now they've returned with an invasion-fleet to wipe out humanity in repayment! Mickey tricks them into believing that humanity has developed Psychic Powers in the meantime, by taking advantage of the knowledge that Abu Simpel contains their landing-beacon, and claiming that he was using his powers to affect their advanced navigation-equipment. Sure enough, the UFO crashes on approach, and the aliens are scared off. How did it work? Well, Abu Simpel was moved - in a massive engineering undertaking - back in the 1980s, to protect it from a flooding. The new location obviously screwed up the navigational calculations, but the aliens obviously didn't imagine that anyone would tear down something that huge, stone for stone, and then rebuild it exactly identically somewhere nearby.
 * Donald Duck: One story has Scrooge McDuck make a bunch of money by producing and selling semi-sentient 'growing' cars to people. Things are already falling appart, however, when an alien race arrives and hits the planet with a shrink-ray, designed to leave the people of the planet helpless when the invasion-fleet shows up later... but since said aliens happens to be a race of sentient cars, they mistook the growing cars for the dominant species of Earth, and shrunk those instead of the people. Since there's Negative Continuity, however, we never get to see the invasion-fleet show up, but presumably, they'd be in for a nasty surprise...
 * In the XXXenophile story "My Favorite Oitling", the human explorer convinces the Martian warrior women that the huge suit of power armour that arrives to rescue him is a typical Earth female and so Earth will be too tough to invade.

Literature

 * "The Best Policy" by Randall Garrett: Alien advance scouts kidnap a human, stick him in a Lie Detector, and order him to describe Earth. He manages to give them a description in which every sentence is technically true, but the overall effect is a misleading picture of humans who possess immense, even supernatural powers, and the aliens are frightened off.
 * "Ginger's Secret Weapons" by Peter Wingham, originally published in Story Teller magazine: A schoolboy named Ginger is abducted by froglike aliens, and uses the contents of his pockets—which include itching powder, a small rubber ball, and a slingshot—to reduce them to a state where they're promising they'll go away and never return if he'll only just, please, leave them alone.
 * The Demon Breed by James H. Schmitz: Aliens planning an invasion capture a remote scientific outpost to study what humanity is made of. The scientist at the outpost tries to sell them a story about humanity having secret mutant warlord protectors, the Tuvela. This being a more dramatic take on the trope, the aliens don't immediately buy it, even though it was their theory in the first place, and it's up to the heroine to cause enough of the right kind of trouble to persuade them it's true. Part of her way of doing it is to make herself think as if it is true, so when she delivers an ultimatum, the narrative says it's a Tuvela speaking.
 * And inverted in Geest Gun by the same author: the advance recon party has come and gone—finding the world easy pickings—and the protagonists essentially have to blackmail the government into getting the fleet ready.
 * Spider Robinson's Callahan's Crosstime Saloon story "The Guy With The Eyes": An alien is the advance scout for a race of extremely powerful aliens who plan to destroy the Earth. He comes up with the idea to render himself unconscious so the other aliens won't receive a transmission from him. Since he is exceptionally powerful himself, they'll conclude that humanity is too dangerous to attack and leave us alone.
 * An inversion of sorts occurs in "Victory Unintentional" by Isaac Asimov: Humans send a team of highly advanced robots to negotiate with hostile aliens living on Jupiter. In order to be able to survive in the extremely high gravity, the robots have been built to be extremely strong and durable (the aliens themselves function more like deep sea fish and maintain their internal pressure the same as the outside in order to avoid being crushed). By the end of the story, the aliens surrender to humans, and after some confusion it's realised the humans never told them they were sending robots, leading them to assume that humans are a race of super-powered indestructible metallic beings.
 * A short story called "Master Race" had the conquistadorial aliens' mighty armada of miles-long starships flee the Solar System in terror after their scout swiped some comic books from a boy's treehouse and the aliens concluded these were historical records of the awesome powers of humanity.
 * In the short story Iron Inferno from the Warhammer 40000 anthology Fear the Alien, a Lord General, of the PDF of a conspicuously Japanese system, made a ploy against the Waaagh! that had just made planetfall. The plan was an elaborate deception to convince a vanguard force that a poorly defended hive was a veritable fortress with many more defenses and men guarding it than there actually were. After a brief battle, the deception had indeed worked, but the Lord General was horrified that his goal met failure. Because of his inexperience with Orks, he didn't foresee that not only would they not avoid a costly and hard-fought battle, but they would jump right at it.
 * A story from an old issue of Boys' Life has a young boy doing this to a team of Martian scouts completely by accident. He's just moved into the neighborhood and thinks the scouts are neighbor kids playing spaceman, and decides to play along. Through a series of contrived coincidences he ends accidentally convincing the Martians that that all of humanity is fearless and morally incorruptible, and that humans far outmatch the Martians technologically. In the end the Martians decide to invade another planet.

Live Action TV

 * A version of this is attempted by Joshua in First Wave in order to prevent the invasion of Earth by the Gua or, at least, forestall the Second Wave. He continually brings up the experiment that resulted in Cade Foster (AKA Subject 117) becoming their greatest enemy. Joshua argues that, if every 117th human is The Determinator like Foster, then the invasion is doomed from the start or, at least, will be a Pyrrhic Victory. Joshua does not succeed in cancelling the invasion, but it is put off indefinitely.

Newspaper Comics

 * The Phantom, the 1970s story "The Blue Giant": Alien advance scouts land to check out whether humans are a good prospect for invasion. After encountering the Phantom, they decide that if all humans are like him, the planet's best left alone.
 * Calvin uses a variant of this, telling them he is the Supreme Potentate of the Earth when the aliens ask him where he is, then selling them the planet in exchange for them completing his homework. Additionally, he forgets to tell them about winter.

Video Games

 * Attempted in Iji by the invading Tasen, who are themselves fleeing a species of genocidal aliens known as the Komato - when they bring down a Komato scout team, they realize that when said scouts don't report in, their commanders will know that the Tasen are hiding on Earth, and curb stomping will ensue. So, they fake a report from the scout team claiming that before they went down, their planetary scans failed to reveal any sign of Tasen habitation.

Western Animation

 * Superboy (1960s Filmation series), episode: "Operation Counter Invasion": Aliens land and want to take over, and Superboy bluffs them with things like "Fly? Of course I fly, doesn't everyone?"
 * Garfield and Friends: Garfield tells the alien scout that Earthlings eat a lot of food and sleep a lot. The scout analyzes Garfield and becomes convinced that's Garfield telling the truth, then concludes that Earthlings will make terrible slaves.
 * In Lilo and Stitch, . It's a big Brick Joke, the trope part of this comes right at the end of the movie.
 * An episode of Arthur, "The Contest", has a short story by Buster called "The Day the Earth Was Saved" (which contains several references to South Park, including the animation style) where aliens capture Arthur with the intent of eating him but first subject him to brief medical testing. They then toss him out of their spaceship and leave Earth, assuming all humans are as high in cholesterol as Arthur is. The real Arthur is not amused.

Real Life

 * Nathan Bedford Forrest, a Confederate general during the American Civil War, was quite fond of this trope. In one instance he had his soldiers march in plain sight of the enemy, then as soon as they were out of view they would loop back around, making his forces look much larger than they really were. He was also known to make fake cannons out of logs.