For every item in your inventory when you visit a merchant, that price is saved and you can refer to it when not at the vendor. 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, my skinning knife, poisons, bandages, etc which I never want to drop or sell.

/inventorylist hide <itemname>
/inventorylist show <itemname>
/inventorylist list

Note that itemname is CASE-SENSITIVE!