iPhone 5 - How can I know if Imei will be clean forever?

By reading the previous posts I came to the conclusion that even I buy an used iPhone and it gets blacklisted, this list is only valid for US carriers? Context: I'm a brazilian going to NYC in Jan and want to buy an iPhone 5.

There is a big problem right now with people going into Apple stores and Cell phone stores, buying as many iPhones as they can get and then immediately selling them. They almost always then report the phones stolen to get a new one from the carrier's insurance company--which they subsequently resell.

They never pay their bill and tend to work the cycle as quickly as possible.

Anyone selling an unused or new in box iPhone on craigslist or ebay should be treated with caution.

If you must buy on craigslist or ebay, look for phones that have actually been used by the seller. This still can't guarantee that the phone isn't stolen, but it's probably a safer bet.

Meet at the carrier's store and make sure you take down the contact information of the person who is selling it. If this does happen, you can report them for fraud and the like.

Make sure to take down their license plate number as well.

I bought an iphone 5 through craigslist from someone who I thought was honest and he was from the military. Iphone worked perfectly I even called 611 and let them know the imei so it would tie to my account, they said ok it's on your account now. I have Att gophone btw. Then a few weeks ago I made a phone call and it said this device had been blocked but it still showed Att at the corner just not LTE. So I called Att off a landline and asked what was going on. They said the owner called and said device was reported lost or stolen. I then told them I had bought it off someone and called 611 to add the imei to my account so I was the owner and I never reported my iphone lost or stolen, then in the middle of my convo the phone died. I was without a phone for 3 days...then I called off a landline, told them I met someone on craigslist was testing phone and sim got blocked. They unblocked my sim. Everything worked for about 30 mins then again the same thing happened. I called back gave same story this time I turned cellular data off, but left service on...I haven't got cut off ever since. I haven't turned cellular data on unless I needed it then shut it right off. I've also check online for blacklist checks and it said it was clean. Someone said if you put your sim in the phone and use it it activates on your account and someone calling it in lost or stolen makes it a bad practice on Att. What can I do? What should I do?