Some items you can also sell on the exchange for a decent price. At low (LT) level, you can possibly get away with selling common gear for more than what the vendors will give you if you sell for less than what the vendors sell it for. Uncommon + items you can boost for a little more. The catch is that you will not get instant EC. I sometimes browse what is being sold if the exchange isn't saturated with the item, I'll put it up.
All starbases (and some planets) have vendors. Here is an interactive map of earth space dock. You can use it to see where all the vendors are. On that page there is also links to the maps of other stations. No matter where you go you will only get 50%.
That's really cool.
Also, Ferengi sell at 30%, so don't sell to them! (I know it was mentioned that they are stingy, but there were no exact values)