how would he prove to ebay that it was working ?
its now his word against the buyers. and given that ebay lean towards the buyer everytime it would be a waste of time to try and pursue this.
chalk it up to experience and move on.
Proving it's working is simply stress testing it, showing a video of it and the test results. It simply shows eBay that they and you as a seller were scammed or that the return was under false pretense. I never gave up when I was scammed as a seller and as a buyer, they usually side with common sense. When a seller sold me a fake damaged motherboard, I told eBay, and I also included pictures and copies of the correspondence and lies the seller made. eBay didn't side with me because they side with buyers, they sided with me because I showed them the guy was a lying sack of excrement selling faulty motherboards and not the actual product he had taken a picture of by taking my own pictures showing small variations not on the listing photos.
It may well be a waste of time, but lets face it, the OP phones a fake repair shop who claimed they got some non tech person to fit and test a 5090. This sounds a bit "Jackanory tell me a story" to me, someone somewhere is telling tall tales. A lot of time wasted chasing up some interesting themes.
Reporting such false returns is part of eBay's policy, you can look it up. If you do nothing your simply letting someone bend you over, if you report it there is at least a record that will be noted and affect the buyer.
Quote
eBay's policy focus:
The eBay Money Back Guarantee is designed to protect buyers when items are faulty or not as described. However, sellers are protected by the policy when a buyer returns an item in a different condition than received or if they are found to be abusing the process.
They state, issue a full refund, do NOT deduct, and tell you to take photo's or videos that dispute the buyer claim.
Report the buyer:
After issuing the refund, go to the item in your Returns dashboard and select Report Problem to report the buyer for misusing the returns process.
I lost interest when the story started taking weird curves, especially at the point of phoning strange repair shops to chase up something one of their customers claimed with regard to a PC with no PSU where a repair shop has the non tech who gets to play around with a 5090 to see if he can get it working for a customer paying for tech services. Not sure how UK GDPR laws are affected by strangers phoning a shop to ask about what a guy buying stuff off eBay was claiming. Frankly no genuine retail business should be disclosing any information on customers or what they were having done. The fact that he has it on record that the buyer took it to someone where a non qualified staff member tinkered with it would be grounds for a serious questioning about what was returned. I would certainly be providing those details to eBay and questioning why I am being asked to accept a product that was worked on by a non qualified staff member in a repair shop.
I mean can you imagine buying something, deciding you do not want it despite the UK right to return policy, but instead taking it to a garage where they let the cleaner try and fix what isn't broken?
So not only was the claim made it was not working, there is also a claim that the buyer had someone work on the GPU, and one of those people was not qualified to do so.
Not sure what eBay policy is on sold goods being tinkered with then returned as not working, but usually a buyer claiming to have something worked on is a red flag as it then disqualifies their own return policy as the sold item cannot be returned as purchased due to having been used and worked on.