One of my favorite add-ons is Sell-O-Matic. Download it at curse-gaming.com. I haven't found it on some of the other mod websites that I check.
Essentially what it does is instantly sell all your vendor trash (grey text items) any time you bring up a vendor's interface. Here's a picture of an item with grey text (meaning it has no value to anyone for any quest or recipe):
Won't Sell Darkmoon Faire Items
As you may know, there are a few items that, although they have grey text, they are wanted by NPCs at the Darkmoon Faire. The good news is that Sell-O-matic won't sell these automatically:
Add items to the Sell List:
One additional thing you can do is add any item to the list. Just type "/som add" then shift click the item you want to add and hit enter. Sell-O-matic will now also sell this whenever you visit a vendor. The command looks like this:
/som add [Raw Rainbow Fin Albacore]Notice how it now tells me it will automatically be sold as I mouse over the item:
The settings you enter are effective for all of your characters. If you have a character who happens to WANT that item then you can remedy the situation by just visiting the "buy back" portion of the vendor's interface and purchase it back. Find more information about buying back items on the worldofwarcraft Merchant page.
If you think this mod is for you download it at curse-gaming.com. I'm glad I did.