One of my favorite little add-ons, AutoProfit does one thing and it does it well. When you click the stacks-o-gold icon that it adds to vendor windows, it automagically sells all your gray items (aka vendor trash) to the vendor. It's customizable, so you can tell it to hold on to certain gray items or conversely to sell certain non-gray items (Fish Oil, anyone?). It also tells you how much all your junk loot adds up to when you mouse over the sell-all button, and let me tell you, there's nothing like making a few gold in junk rocks off elementals in the Badlands to make late-40s grinding feel a little more pleasant.
Since getting this little baby, manually clicking on 37 items after having moused over my entire inventory to seem what's salable seems like such a chore. This is one of those "should be in the default UI" things. Download it at Curse; it's updated for 2.0.1.