Several months ago, I purchased a like-new Galaxy S4 on, a well-known, reasonably safe used mobile device site. The phone works perfectly, and I continued to use it on T-Mobile without any trouble for around 5 months. Then, seemingly for no reason, my connection to T-Mobile disappeared. I tried everything to get it back, from toggling Airplane mode, running a factory reset, and even installing alternate operating systems, but nothing worked. So I went to T-Mobile. I was informed that my phone had been purchased on a contract, and that it had been permanently blacklisted on their network. It had worked perfectly for several months, and I had no way of contacting the original owner, who used fake info when selling it to the person I bought it from, but they said they couldn’t remove it from the blacklist. This would be just fine if they had blacklisted it right when the first or second payment on the contract was missed, but T-Mobile waited until over 6 months after the missed payments began before taking any action, by which time the phone had already been passed on to a new owner, twice. This left me with a perfectly functioning, yet useless, high-end phone. I was able to get the phone unlocked and working on AT&T, but with it never able to work on T-Mobile’s network again, the value had been greatly reduced. I’ve since purchased a new phone, and will never deal with a phone contract or used device again, because it’s just not worth it.

TL;DR: T-Mobile is crap, used phones are too.