I unknowingly bought a stolen item from a seller

[range0rover]

On Feb 10, 2016 I bought a Cable modem/router from an eBay seller.  I started having issues with the modem about 3-4 weeks ago and today I finally contacted Xfinity about the problem. 

They told me to bring the modem in to the service center and they would add it to my account.  when I got there they took a look and informed me that the modem was stolen and that I would need a new modem. 

They also said that they wouldnt confiscate the modem from me because I might be able to get a return/refund, however, the person that lost the modem was still having to pay for it because it was actually  Xfinity's property. 

I would like to get a refund from the eBay seller but I dont want the guy that lost it to have to pay for it either.

Should I contact the seller directly?  Report the seller for selling stolen property?  Contact eBay for intervention???

Any suggestions on the best way to handle this?