You can get ok deals on EBay if you know what you are looking for. Some people have no idea what stuff is worth, and pay way to much for it. Ex. I have seen used fluval filters going for over 50.00??? If you check a sellers feedback, and they have obviously sold lots of items to lots of satisfied customers, chances are you will get a good product. If a seller has lots of oddball stuff listed, like a garage sale beware, also watch out for the "to good to be true" auctions. If some has a dutch auctions for 100 10K MH bulbs for 25.00 each, they are scamming someone.
Just watch you who buy from, you aren't going to get stuff for 1/10 of what it is really worth. Don't pay by money order, since they is no tracking mechanism, don't send payments to PO boxes.
hope that helps.