Can i use a iphone i bought online before its blacklisted?

I decided to buy an iPhone 5 online to save some money. I checked the phones IMEI number online and it came up clean, not blacklisted. As i did more research I read that some people buy iphones that are not blacklisted at the time but then later become blacklisted. Now i'm worried that if the phone i got was lost and not reported yet but becomes blacklisted in the near future and i put my sim in it while it's clean. If i activate this phone (with att) and it later becomes blacklisted what happens? What should i do, should i trust that it's clean?

Added (1). Also could it be that the original owner did not block the phone so they can catch the first person who activates it? And use the IMEI to trace it? It was a sketchy sale. I hope i'm not setting myself up for trouble or a waste of money.

If it has come from a reputable seller, a company one, you should be fine. I bought and ipad off ebay and its perfectly fine. As long as the seller as more than 3 stars, has more good reviews than bad, and the phone is perfect condition, consider yourself having got a great deal.