The seller may have reported the device as lost or stolen. T-mobile does not block IMEI unless it was for that reason, or if it was not paid off. That would be my guess. Also this is a public forum so you should not share things like the IMEI number for your device. If you want to work with a T-mobile rep to get more details I would suggest messaging them over social media. I have linked the contact us page here Contact Us
Could mean it's blocked from using their SIM (e.g. SIM locked).
Because, ya know, apparently you buy a device "unlocked" from Apple, but it becomes SIM locked to the first carrier it gets used on. Some folks don't realize that.
Either way, the answer is you have to work it out with the seller. You didn't purchase anything from T-Mobile, so T-Mobile cannot resolve anything.