Personally, if you're buying used, do what you'd do with a used car, get it checked out by someone who knows what they're doing. Maybe the local hobby shop will do it for a fee. With no experience, I'd suggest you stay away from buying it online without ever seeing it fly.

I'd personally suggest you go to the local flying field and ask if anyone wants to sell a 1-3 year old heli with not too many gallons through the engine. I know that most of the guys I fly with have 2-8 helis and would be willing to sell one for a fair price to a new hobbyist.