Free add-on for WoW that remembers the selling price for items
Version: 42SellValue is an add-on for World of Warcraft that remember the selling prie for every item in your inventory when you visit a merchant.
Operating System: Mac OS X
Also has a bindable /inventorylist window with all your stuff and sell prices, which also functions like a normal bag (shift click link, right click equip, etc).
In addition to using /inventorylist to toggle the frame display you can also prevent certain items from ever showing up on the list. For example, your skinning knife, bandages, poisons, etc which you'll never want to drop or sell.