So, if im going to buy an account i should first have 100% sure that the Guy is legit and wont try to recover it later?
So, Even if i had changed the e-mail/Pass and take Screen shots and all, he still could e-mail areia to prove he's the owner and try to get the account back? (Probably resulting in a Ban) Thats what happened?
I doubt they are banning for account sharing. They will only ban you if they actually go into their logs and check out your account to see two different I.Ps continuously switching between the account. This means you and another person are sharing the account. But if you traded/bought an account, you should be the only one going on it. The old owner's I.P should no longer be an active I.P linked to the account.
Another possible reason for these bans is that the previous owner is trying to recover the account by requesting a ban then doing a ban appeal with the original email and other information such as AP transactions, etc. Or they reported to Aeria that the account is being shared.
TL;DR : NO, I DO NOT THINK AERIA IS BANNING OR ACCOUNT SHARING.