I bought my son a samsung galaxy s4 on ebay back in August. I wanted to specifically use it with Page Plus, so it had to be a verizon phone, with a clean esn. Phone works great! I'm now "shopping" for a phone for myself.
If you decide to go this route, look for a power seller, with very good feedback ratings. The one we used had been in business for years, and had 99.5% positive feedback with over 5000 transactions. They gave us a 2 month return policy for any reason. That at least gave me 2 months of peace of mind.
Make sure it has a clean ESN, and look for a phone that can be used with the carrier you are going to sign on with. Both facts should be stated very clearly in their auction. Make sure you read the details of the auction carefully just so that you have a good understanding of the phones condition. And, as someone already mentioned, if you pay through paypal, you are offered buyer protection. I've had to use buyer protection once, and it is a bit of a hassle, but at least I got my money back, minus shipping charges.