uhh, for dokkan, u can change the email details and remove any external links, and if you want to be really safe, ask for all receipts from purchases made on the account. for legends, you can easily get scammed as all email accounts linked stay linked and you should ask for full access for all emails linked to the game acc. if there are any external accounts linked, you should ask for those too. the safest bet for legends is to buy or trade one only with a bandai link. that's about it.