+1 for Swappa. My T-Mobile business rep actually recommended we use the website, and we have! All of the phones in our office have been purchased of Swappa now. We only had one bad phone, something definitely wrong with the phone as it wouldn't connect to any carrier we tried it on. Swappa.com made the seller make it right in under a week. I've had worse experiences dealing with carriers directly w/r/t bad phones.