Bought a Xfinity Flex TV Box AX061AEI on eBay but it's already registered to an account.

Hi all,

So a simplified version: I bought a 'in-the-box' Xfinity Flex TV Box AX061AEI.  Since it was new and the seller takes returns, I figured no harm, no foul.  

The box IS new but I can't add it to my account because it's registered to an Xfinity account.  The fella at the call center told me to take it to a service center and they may be able to clear it for me to use but after digging a bit deeper, I realize that Comcast/Xfinity likely considers this unit their property and will want it back.  

My question: Should I return it to the seller for money back or since Xfinity knows that I have THIS particular box are they going to try to get it back from ME.  Should I return the box to Xfinity and flame the seller?  I'm particularly looking for advice FROM Xfinity or someone experienced with this type of situation.

Thanks