But it is a lot more of a chance with a T-Mobile device. The OP tried to do everything right by meeting the seller at the T-Mobile store and verifying and activating it onto his account. According to TMONews;
"Thankfully, AT&T, Verizon, and Sprint have a smarter system in place. They will tell you if the IMEI or ESN (the network identification number for the phone) is active on an account by simply calling them. If the phone is released off the account and not reported lost or stolen, you can be most certain the device wont have any issues being activated on a new line and you wont have to worry about your service getting cut because the past owner stopped paying their bill, like on T-Mobile.