I remember an old addon (before BC) that built up a database with the amount that all items in your bags are worth to a vendor. this helped immensly in deciding what items to keep and what to throw away when my bags are full. it simply showed the amount of money every item was worth to a vendor at the bottom of the item's tooltip.

i was wondering if this addon or one like it still exists? i search for a little bit on the forums but i couldnt find a thread about it.

ItemDataCache i think its called

There are a large number of different addons that do this. I personally use the Auctioneer package. The Informant part of that package puts a number of different things into an items tooltip, vendor price is one of them.

Titan Panel - Itemized Deductions (Reborn) does what you want and some more. It uses above mentioned IDC and Informant as databases for prices.