The phone is probably carrier locked or possibly blacklisted or both. In either case only the original owner can resolve the issue so would encourage you if still within the Ebay return/claim window exercise that option.
To determine if blacklisted use this site, enter the IMEI number and if it says ok for use on T-Mobile network then the device is clean and not blacklisted. Devices are blacklisted for a number of reasons like it is reported stolen, lost or if it still has a finance contract that has not been paid in full.
If and only if the device shows eligible for use on the T-Mobile network then the device is carrier locked (SIM lock) for use on one carriers network and only the original owner can request the unlock code if they meet all the critera. There are no exceptions it has to be the original owner that purchased and had an account with T-Mobile US. Here are those requirements;