My iPhone is locked to T-Mobile account

Nothing.

• You cannot remove a Carrier Lock, that is only done with the cell provider that locked it. You may find that the cell provider will not give you any information about the contractual obligations with the original owner that agreed to the lock, and claim it is for privacy reasons. They are not required to unlock it for you.
• The Activation Lock that requires the previous owners credentials cannot be bypassed. This is just 2 of the reasons why it is risky buying any second hand phone when there is no option of returning it to the seller.

The All Too Common SAD Reality of Buying … - Apple Community

The iPhone is locked to the previous owner's T-Mobile account.

T-Mobile will not unlock the phone for you, because the phone is not assigned to you.

Your only hope would be to contact the person that you bought the phone from and have them contact T-Mobile to have the phone unlocked.

As you might imagine, there is not much incentive for the former owner to do this, since they already have your money and would have nothing further to gain by having the phone unlocked.

Your best bet would be to try or a refund, if the seller has not already disappeared.

Click on the link to read Apple's support article on how to unlock your iPhone for use with a different carrier. - How to unlock your iPhone for use with a different carrier - Apple Support - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Click on this link for iPhone: Wireless carrier support and features in the USA and Canada - Wireless carrier support and features for iPhone in the United States and Canada - Apple Support - including a list of which ones offer unlocking, plus links to other countries.

Note that Carrier Lock and Activation Lock are not the same issue.