Can i use an iPhone 5s at&t?

I bought an iphone 5s. I didn't know the person was on a two year contract. They broke it. So does that mean if I put my own at&t sim or at&t compatible sim it won't work? Apparently they JUST bought the phone and sold it to me just a few days later.

As long as they don't have it with their account anymore. Just put in your SIM and try it. Don't you think that would be the best to try before posting?

If they stop paying bills, your phone will be blocked. And if the phone gets blocked, that can be fixed too.