Infinite Stock for Sale

The video game merchant counterpart to the ever popular magic satchel mechanic. Essentially, it's when even after The Hero has purchased 99 or so Potions/Swords/etc., the item shop will just keep right on selling them. Arguably considered to be an Acceptable Break From Reality due to the potential frustration that players may endure should there ever be a limit to how much stuff you can buy from a single store alongside with designers having to keep track of the amount of each item in every store available. Occasional subversions will have certain items and weapons that can only be purchased once, such as a plot-specific item or an exceptionally rare weapon, and have everything else be bought infinitely to distill the frustration.

Compare Infinite Supplies, which is a non-gaming variant on this.

Note: Please limit examples to aversions and inversions, since this trope tends to have a habit of being everywhere.

Exceptions

In Rogue Galaxy, many shops have limited quantities of certain items, and they are indicated by a quantity number on the shop screen. (This doesn't stop other shops later on from stocking the same items in unlimited quantity)
The X series uses limited stock as a feature, creating an in-game economy that the player can participate in.
In The Elder Scrolls, shopkeepers have a limited inventory (although they might sell infinite amounts of some very basic items). In fact many have a limited amount of gold when selling (meaning you might have to barter a bit when selling a valuable item).
In Recettear you always have a finite number of items to sell (since you're limited to what you find or buy wholesale). The highest level of items you can buy at wholesale price from the Merchant's Guild and some of the items in the market always tend to be only available in limited numbers as well.
Each shop in Spelunky sells exactly four items, which makes sense since most of the items give their respective effects for the duration of the entire game.
Stores in Baten Kaitos had limited items, though after a while they would restock.
In Dungeon Crawl, trope averted: every shop has a limited number of items, usually 6-8, in lettered slots. Every slot holds just one item (or a set that must be bought all at once), and once an item is bought it's gone forever, so if you should run into a food shop, it's plausible it holds merely eight bread rations and will never, ever restock. Combined with the rarity of nonperishable food outside shops, and a pretty harsh hunger system, this was intentionally done to discourage players from boring level grind strategies.
Fallout games in general are aversions, as shops only have limited stock. Not only that, but the shop owners had limited money as well, meaning that if you're selling items to them, you could only sell them so much before they were out of money. This can lead to situations where, if you have a high-value item, you can't sell it to some shops without taking a loss (i.e. it might be worth 1000 caps but the shop keep only has 400 on hand).
The raccoon merchant in Super Mario Sunshine trades Shine Sprites for blue coins, both of which are in limited quantity. When he's sold the last one, he wonders what he'll do for a living now.
Generally, in The Legend of Zelda games some shops will sell items that Link can use forever, such as new armor or pieces of heart. Once these are gone, they will sometimes be replaced with different wares but more often they will be sold out forever, denoted by a wooden sign with an X painted on it. This is just as unrealistic in the opposite direction, as it means these shops only have one of the given item and had no prospects of selling them to anyone else. Items that can be used or lost, like potions or ammo, will never run out and otherwise infinite use items that can be lost (such as a wooden shield that was destroyed) will be back in stock after they are lost.
The shops in Disgaea carry a limited selection, but replenish in the three seconds it takes to leave the shop and re-enter.
Averted in Neverwinter Nights 2. Not only do merchants have a limited stock (apart from very basic items such as +1 Healing Kits), their money supply never resets.
Dragon Age averts it for anything except basic crafting components, though the merchants appear to have infinite funds.
Averted in Angband and its variants. The stores have limited amounts of supplies, though they will restock eventually. It is also possible for items to restock with zero of something basic on the shelves.
Pokémon normally plays this trope straight, but in Gold and Silver, there’s a bargain shop that only appears Monday mornings, where you can buy Vendor Trash below cost to later sell at a profit—“but only one of each item.”