Limited Supply (WoW)  

Some vendor only hold a limited supply of a certain item. This item will appear with a small number in the bottom right corner of the icon to show you how much are being held. Once the item has been sold out, it takes a few hours for a vendor to 're-stock', during which time the item will not be purchasable.

Items that are out of stock do not show at all in the vendor's list.

World of Warcraft Wikibase™