Greg bought a used Samsung Galaxy S2 and it worked for a month before a previous owner blacklisted it, preventing him from using it. Leo says Greg likely bought a stolen phone. He can unlock the phone and that would allow it to be reactivated again. Then Greg can put an AT&T SIM in it and use it again, but it's dead with T-Mobile.
Since they T-Mobile store wouldn't do anything for him because the phone was blacklisted, Leo recommends going to Gazelle's eBay store and buying it there. At least Greg would be protected then. This is why Leo doesn't generally recommend people buy phones from Craigslist.
Leo also says if he goes to T-Mobile, he should just ask them for a free Galaxy S2 with sign up. He might be able to get them to do that.