Have an iphone 6 plus that a relative gave to me as a second line on his AT&T contract?

I have an iphone 6 plus that a relative gave to me as a second line on his AT&T contract. He disappeared about 2 months later and I have not heard from him since and its now been about 8 months. The phone was on the 24 month payment plan to pay it off. Now I want to know, can I sell this phone?

Call and ask the service provider. The person you sell the phone to may not be able to get service for it and if that happens they will come back to you.

Whoever pays that AT&T bill is paying for that phone for 24 months period. If the 24 months is not up, you will still pay for the phone, but the next owner may or may not experience trouble activating it on a different account