With Android users, there is a way to link your bank to your Google account in order to make in-game purchases in apps. When lending your account out for services or selling it, be sure that you're protecting your information. Below is a link to the payment methods that can be on your google account:

Add payment methods to your Google Account - Google Payments Help

Unlink any payments and make sure you change your password to protect yourself as much as possible when lending/selling your pokemon account. Remember, when lending an account the party performing the service has access to your google account (if you use login with google), not just your Pokemon Go account.

When buying an account, assume that the google/pokemon club account is able to be recovered. Exercise extreme caution with what information you add to said account.