You are mistaken that there is anything you can do to ABSOLUTELY prevent them from suing you. But understand, it is very easy to threaten to sue, a bit more involved to actually file suit, and prevailing is going to determine the facts.
Certainly, offer them a complete refund (in exchange for the merchandise) and be done with the turkeys.
Understand you're both breaking the law on the use of the SEN accounts. You can't pimp out your account with the unit you sold.
I'm having a hard time understanding how any PS4 is worth $1000 or even $500.

