Selling a phone not fully paid off

Until the phone is paid off it is locked to AT&T, and cannot be used on another AT&t account either. Selling it to someone else would be scamming them because they would not be able to use the phone.

AT&T is now enforcing its unpaid devices policy. This means you will not be able to use the phone on any account other than the one that is still paying for it, yours.

https://www.att.com/support/article/wireless/KM1509747/

Doing an IMEI check on swappa only shows that the device is not blacklisted.

Swappa will not allow you to list a device that is not paid off.

Whoever would buy the phone will not be able to unlock it… once they find that out they may want to return the phone and have you refund (return) their cost. 

To unlock the phone before selling it, you would need to pay the off completely… no payment plan.

I am a little confused on this because I have read they can still use it on the att network just not others 

And the IMEI is clear on Swappa 

They could used a locked phone on another AT&T account, but not a phone that is not fully paid off.

To address your other question, you can stop using it and start using a different phone, but you wouldn't be able to start a new AT&T installment plan on that line.