DirecTV can't take the customer's equipment just because the customer once subscribed to the service, any more than Comcast can take your TV or your ISP can take your cable or DSL modem.
If the box was leased by the "seller", though, DirecTV still owns it. They can demand it back, but since you weren't the leasee, they can't demand a fee from you, and if you refuse to give it back they'll have to proceed through the courts to recover it. However, they're lawyers are bigger than you and if you force them to do this they'll probably end up owning your house as well. There ain't no justice.
In any case they won't re-activate the box, so it's useless even if you don't return it.