In general, of course, the sale of an account is not something that contradicts the ToS of an online casino, and this may lead to claims from the platform to the buyer, because here you will probably have to use a VPN, so it may cause blocking in the future.
Is that a mistake? Because sales of betting account contradicts the ToS of many gambling sites, including casinos, only few gambling sites do not include that in their ToS. Just like you said, some people may buy an account, use VPN and later their account will be blocked because some gambling sites included the forbidden of VPN in their ToS. Before using any gambling site, the ToS should well be fully read.