is buying accounts bannable?

yes it is definitely bannable.

It's a direct breach of the ToU, which basically states that "sharing of accounts in any form" is ban-able.

Submitting a ticket to Support is probably the best way to address such situations.

Yes, so is account sharing.

IMO, buying a high elo account is millions times worse than simply buying an unranked 30 account so you don't ruin hundreds people's games until you can play ranked.

I mean you need to prove it that's the problem

It is, but its not really going to be actioned on unless the person who bought it stole the account or used third party software on the account.

Its realistically not worth their time dealing with it if there is literally no harm in the transaction.

Buying accounts is definitely a bannable offense, but you'd need some hard proof that it was a bought account and not just like from their KDA or chat logs. If anything you just report for "third party program" and write in description "bought account"