I'd go with ebay. A lot of sellers on amazon with used equipment seem pretty sketchy, and a LOT of them seem to be shady places that don't care if you give them negative feedback. Ebay sellers really care about their feedback, so they do a much better job about describing the items in the first place, and they will work something out with you afterwords if something goes bad, just to keep you happy.
In fact, I'd say I trust ebay more than any other source. You're dealing with people, and their reputations are right there in front of you. Isn't like that at a department store.