If the phone was sold as unlocked file a claim with ebay and if you used PayPal file a claim with them.

Well...  The solution is, you don't meet the requirements and there is no way to verify the device does either.  That means, it will not be SIM unlocked by T-Mobile.

but then in my case there will be no solution? since the seller does not answer me, there is not even a solution that I spend an amount so that I can unlock it?

That has nothing to do with the SIM Unlock Policy requirements and needing to be a current T-Mobile USA customer.  A clean IMEI just means the device isn't blacklisted.  

I did a search for his imei and saw that there are no restrictions, just the block

If you are not the original owner.  How are you sure the device meets all of the SIM Unlock Policy requirements?  Are you a T-Mobile USA customer?  

yes it meets all the requirements but I am not the original owner, what step should I take to resolve this?

You.must be a current T-Mobile USA customer and meet all of the SIM Unlock Policy requirements. 

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

You are not the original owner of the device.  That might complicate things even more.  You might want to reach out to the seller and ask them to request the SIM Unlock, if they are the original owner of the device or return it because it was not as advertised.