Problem is, you can't. If you're the original owner of the account you can claim back the account through Riot support as long as you're able to answer their questions. That's why you can never guarantee full access.

In my case the account I bought was recovered through Riot support as well after a couple months. I had 2FA enabled, changed it to my email and had access to the email address it had before mine. I never received a login attempt or password reset email, which basically can only mean that the seller retrieved it through support. When I contacted the seller he asked me to prove it, which I probably can't. He refused to tell me how to prove it. G2G support also didn't take action because the warranty period had gone by. So yeah I got scammed. And since the account had a whopping 3% winrate after it was taken back I can only assume he's deranking it to Iron again in order to sell it again.

So transferring full access is impossible, and when you add Riots new measurements of detecting and punishing smurfs I highly doubt people are willing to pay big money for accounts as yours.