I had the exact experience selling an iMac on eBay about 10 years ago. Buyer clicked buy it now, my listing was removed, they never paid. They deleted their account shortly after (it was some .ru email address.) eBay had nothing to say other than “try again after X weeks”. Why, so it can just happen again? They admitted they have zero protection against it.
I deleted my account and sold it on Amazon instead. How anyone puts up with eBay I have no idea.