Sold tuner, buyer registered it, now claims it caught their computer on fire

Hi.

I sold my HP Tuner MPVI on ebay. I gave them the application key and other information they asked for. Now they are claiming the tuner never worked and caught their computer on fire! I verified it worked before I listed it. When I sign into my HP Tuner account, it shows the device isn't registered to my account anymore. And I cannot import it because it's registered to another user. So obviously the device was working for them to have registered it to themselves. There were also credits on the device when I sold it. I sold the item as "no returns" because of this, but now they are claiming the device never worked and caught their computer on fire in order to get a refund. Wondering what my options are and if anyone has advice.

What does the application key do? Whoever has the key is the owner of the device? If he able to report the device stolen if he has the key?
Am I able to reclaim any credits he used?
Am I able to register the device back to my account and safely relist it for sale?

Thanks

If he has registered the interface he is the owner and has every option open to an owner. You forfeited your rights when you sold the interface to someone else.

You can register your device back if you are able to retrieve it from its current owner.

5 volts that go through the USB is not going to catch anything on fire. Tell him to go pound sand. It was sold as is, no warranty, no returns.

if he has filed a dispute, talk to a person at ebay and tell them the facts. Good seller feedback on your part goes a long way. I've had a negative feedback left for me removed by doing this. Key point in your favor is that the buyer was able to register it.