[The Procrastinator]

You should be aware of this kinda bug in the game: when a merchant has a LOT of items for sale, the quantity of items may go beyond a certain limit where the last items in the list become invisible. They're still there, but you don't see them. Maybe this is what you're experiencing. To solve the problem, buy all of the cheaper items, like stone bullets and plain arrows and quarrels, and just drop them around. If you're far enough in the game, money won't be a problem, and if you're not, the merchants inventories shouldn't be full anyway. You'll see a whole bunch of hidden items reappearing in the merchant's list.