Unlocked phone cannot find service

I bought an unlocked iphone 4 on craigslist today, when I met the guy he showed it to me and it had service, I cannot remember the carrier name he was using, but it had service. I took it home, restored it and put my AT&T SIM in it. It just switches between searching... and no service. I've tried resetting network settings and restoring again. IMEI check online confirms that it is in fact unlocked, and also when I restore itunes gives me a messages saying congratulation your iPhone is unlocked. The SIM is a working SIM card, I tried it in another iPhone. Why isn't it able to connect to the AT&T carrier?