iPhone 5 unlock question - was purchased from 3rd party

I recently purchased a "used" 4 month old T-Mobile iPhone 5 from a seller on eBay (actually I purchased 2 of them) and put them on my T-Mobile POSTPAID account.  Both are working fine here in the USA.  I will soon be going out of the country on vacation. One of them was already unlocked and I need to unlock the second one. Since I didn't personally purchase it from T-Mobile (the seller did) what do I need to do or to provide to get it SIM unlocked?

Once again, I purchased it from a 3rd party who purchased it from T-Mobile.  I am NOT the original purchaser,

Thanks