@therealjonboy, Unfortunately, it sounds like the person that sold you this phone, had bought it using a payment plan from ATT, but decided to sell it on without fulfilling the payments.
Technically, ATT still owns the phone because the buyer hasn’t completed the payment plan :( Unscrupulous people do that sometimes, and it’s really no legal way around the carrier lock. You could return the phone (I think Marketplace offers that, but FB Marketplace work totally different here in Norway as opposed to the US) to the seller, or you can pay the amount due / owed on the phone, to get the unlock code from ATT. This sucks, for sure.