I recommend going with a refurb model. You'll find them on ebay as well as at stores like GameStop. Buying one used is never a guaranteed deal. It could work, it might not. The worst the seller has to lose is a slice of positive feedback. You can always file PayPal disputes and claims but in the end you'll just get your money back and be at square 1.

With a refurb you get the manufacturer's warranty again, which means even if the refurb is a dud (rare, and in my experience so far, never) you can get it taken care of. I actually bought my refurb XBOX through GameStop on eBay. (They have an account and list things periodically)

Of course the only sure way to get a perfect condition console is to buy one new from a store that you can return it to if you have problems. You'll pay for that luxury though.

I prefer refurbished to used because at least refurbished means everything was checked out and is guaranteed to work. Used just means you are playing Russian Roulette.