There's no set way on how those are handled. It would be on a case by case basis.
Buyer could have done things that increase the risk, such as making purchases right after buying, contacting support, trying to get important info such as account country changed etc. Different platforms have different rules, levels of strictness, ban lengths when that happens. There are just too many factors that could affect the verdict.
Most likely it would be shared responsibility, since both trades are aware that account trading is against the platform's rules. And depending on the above factors, the ratio of responsibility would be shifted.