Selling a locked iPhone

So I recently bought an unlocked iPhone 4 from eBay. I was tinkering with the settings after I used a Back Up Restore and realized it was the wrong one. So...I accidentally wiped it clean again and it now says to install a SIM with no Pin Lock to activate. I am overseas, and the previous carrier was T-Mobile. I talked to stores here and they say in order to unlock it, I need to talk to the seller's provider, but I have also heard that I can just call T-Mobile and they will unlock it for me. The seller has not responded after I told him what happened either.

So I am curious as to, if I can sell the iPhone to someone else who has, or is getting a contract and if they can use this phone. I do not want to sell it and it be unusable.