Lots be said already, so this is an opinion.
When buying a second hand gun, unseen , un-fired it’s a calculated risk - will it shoot ? Anyone not thinking this is being foolish. Hence you calculated the price, the faff to get it to shoot and the when do you walk away and scrap it.
If you expect one hole groups, you have the gun built from a rifle builder / gunsmith with track record.
Or expect some experimentation and tweaking to get mass produced components to shoot what you want it too.
Unless a guarantee was given it’s sold as seen, nothing stopping the new owner asking to put a bore scope down it or asking a gunsmith to look over it when he was looking to buy it.
I’ve spared you a legal discussion of small claims court as it won’t get to that, and it’s not worth it with the sums mentioned for anyone.
If your moral compass is spinning, offer the new owner some solutions, give them some cash back but this is also where you have had no control over how they shoot it, what they have done to it in their control so a line must be drawn.
This is also the reason I get rid of my guns via the RFD, but sold to them - so it’s not my problem, Yes never get as much as privately and sometimes it’s painful as it’s still good kit but it’s no hassle from that point on.
Hope it gets resolved.