During the first uncarrier event, that's when John Legere announced no more contracts - only we still have them. Basically what happened is that you get a new phone with a down payment, and a payment each month for the phone until it's paid off. Pretty good deal right? Only, people think they can still sell those phones like with the regular contracts you get at Verizon, Sprint, AT&T, etc. But the thing is that when you sell a T-Mobile phone that has an EIP (you know, paying a certain amount per month for the phone) that EIP is attached to the phone and not the account. So if I were to sell my LG G2 for instance, which is still being paid off, the person who buys it would be responsible for paying off the rest of the device. Which is where companies like UpgradeSwap are getting burned.
Now I know what you're thinking, you checked the IMEI right? Well I did a test on Swappa's free IMEI tool (where you can see if an IMEI has been blacklisted, etc) and the IMEI of a device on an EIP will show up as being perfectly fine and that it will activate. But if the seller decides to stop paying their bill, your new phone can turn into a paperweight, and that's why UpgradeSwap is no longer purchasing T-Mobile devices. It sucks, as a T-Mobile customer I know I buy and sell a lot of phones on Swappa and other services like that as well. And with so many people doing the financing for their device, it was bound to happen.
With T-Mobile being the "uncarrier" I wouldn't be surprised to see Legere do something to fix this, if he can indeed fix it. If you want to read UpgradeSwap's entire blog post they put up yesterday, we've linked that down below in the sources for those interested.