I got my reef tank used and couldn't be happier, being a teenager if I hadn't been able to pick the whole set up so cheaply I wouldn't have a marine tank. I got MH and actinic lights built into the hood and wired professionally by an electrician, an entirely custom made, custom designed tank, hood and stand with a rather unique filtration system complete with a professional custom made filter and skimmer, as I said couldn't be happier, the guy also through in 20kg of cured live rock. I paid $250 for it and the equipment alone would be worth a grand, let alone the liverock.
I have no problems with used, but always ensure it is leak tested before you take, never take it just on the sayso of the owner, I always make em fill it with water and send me a pic before I pay...