Same as others: Warranty and assurance of a working product over next period of X months (for price paid).
Not everyone has Member Marketplace where things are more vetted (and sellers who will be more likely to help you in a warranty situation). Therefore you're stuck with the bay and other locales for second hand gpus. As others have said, you have no idea if there's been any excessive work applied to said gpu's or in what condition; since we all know prolonged exposure to excessive heat could damage the card in irrevocable ways. It's just their say on it's use in history. No way to prove it. Therefore, having a warranty gives me a period where I know the card can be working or remade to work for the price I put out for it. This can't be done with second hand gpus in most cases.
£200 for a second hand GPU that lasts 1 month and no warranty is £200 a month, that's not worth it. (Worst case scenario) But if it does last, great. But unfortunately there's no net underneath on the off chance it does fail (in the first month; making it the worst case scenario).
£300 for a new GPU with 36 months warrant is £8.30 a month minimum guaranteed and potentially longer if a failure happens close to end of warranty period. That safety net of the warranty protects the investment you make into the GPU for the minimum period of that original warranty.
The chance for a card failure may seem low. But it's the same as winning the lottery. If it happens, it's you. Doesn't matter the odds. Therefore, I prefer the new card with warranty.