Ebay is full of scammers. I bought a 3080 and got burned for £1000, despite the so called "money back garuntee" Ebay customer service literally trolling me.
There are some dodgy sellers but also buyers. a lot of the Ebay gpu's are being bid on by buyers who only have between 0 to 2 feedback. Not sure what's going on there but most probably scammers. I have seen some signs of the madness abating though. 5700XT's were selling for £600 last week but now they appear to be topping out at around £540. There are a few with over £580 bids but the bidders look dodgy with the aforementioned 0-2 feedback.