Ebay lets you search for 'sold listings' -- when looking up "Parts only/faulty 980ti" it seems they go for between £40-80 depending on the model and condition.
Better than a slap in the face tbh.
I'm assuming this isn't against Forum T&C since he's talking about selling a faulty item.
As for what GPU you should aim for, keep an eye on OCUK's new forum section for "Member Deals" if you have access to it. With a little luck you should be able to get your hands on something like a 3060ti or 6700XT for close to MSRP.