I'm surprised you even got that for your 3090, didn't you smash it up with a hammer before selling it ?
Seriously now...
You would be mad to sell a 3080 for £500 right now with what is going on in the market.. Anyone with anything higher has zero reason to throw good money on any of these gpus from Nvidia and AMD, if you like feeding the devil and updating every generation then well expect to keep feeding him till he eats you all away.
This generation is to me one of the worst ever I have seen in all my life for pricing. Also this is when I was paying £600-800 for the best gpus over 20 years ago and people were laughing at me back then for doing that and saying buy a console or an all in one pc.
Nvidia and AMD are soon going back to that market style of high prices and low demand as it was all them years back as there were very few into pc's and gaming then and so was expected with the low sales that prices would have been that high but even then we were getting double or more the performance each generation and a generation was only 6 months in some cases to a year not 2 years or more to get 35% on average better performance at now even more silly pricing in the world we live in right now where people can't even pay their bills.
These companies have lost the plot really and anyone that doesn't see that is well blind.