unlooking help

Hey everyone I need some help. I bought an ATT iphone 4s of ebay and the seller said it was unlocked. However, when I put my tmobile microsim card I got an "INVALID SIM" message. This is really bad as I had to get a microsim just to use this phone and it cost me 20 bucks plus another 10 bucks for a case, also my old phone uses a regular sim card so I cant just switch back to that now. To make things harder the iphone was running ios 7.02 when I got it and I dont know of any way to unlock it with this version. So my question is does anyone know of a way to unlock a iphone 4s running ios 7?