It can be because the person want to avoid KYC.
I don't think this reason should make anyone want to buy a casino account. KYC is not only done once; he might encounter another KYC verification in the future and may be unable to pass through that stage successfully. This obviously means that he has wasted money on something he could not avoid.
For me, I think one reason that is very much understandable is that he is trying to skip the lower levels and get to the higher level without going through the process of levelling up.
If this is not the reason, then I fear that the account could be used for impersonation or something worse. By the way, I have never heard of people spending money to buy casino accounts. Gambling is unpredictable, so why spend money to get a casino account? The supposed buyer has his reasons, and whatever those reasons are, they must be beneficial to him.