after looking for some tmobile sgs4's with n4zty i have seen some concerning information on some of these devices.
"It'll go about the deposit Michael, just like now, if their credit is bad their deposit will be high, besides we'd still block their imei due to fraud, they did that one time a lady came in to my store and tried to activate a phone on pre paid, the imei was locked, I looked it up and bam customer owed 1,600+-$ in her contract account, she said she bought the phone herself, not through t-mobile, so I called tech support and retail customer service (yeah we have our own retail line) and nope she got the phone subsidized from us, so I think that either way they'd make their money."
that was in regard to what happens if a person using uncarrier just stops paying, swappa has a few listings of people asking if teh phone is paid off. whats worse is they can stop using their tmobile service, pull teh sim out and the imei is free and clean, 30 days after a missed payment... well sucks to be you if this is true..
if this is the case then i would strongly suggest to be very careful when purchasing used/"new" tmobile devices from third parties