If the buyer has modified the item then I think they've stepped outside the coverage of eBay's MBG - you can't modify an item and then try to return it as faulty whether it's a paint job or something more substantial.

Best to get onto eBay's chat support via the 'Help' menu and have them confirm whether this is the case. Have them read the messages exchanged between you and the buyer.

They might need to put you through to the returns team who would better understand.

But for all you know, if the item is indeed faulty it could be due to mishandling or liquid damage when it was painted.