If you are using the Credits module then every you can take a percentage off everytime credits are cashed in. Its a great source of revenue to help maintain server fees etc, but it also puts all the responsibility of disputes and refunds on you as the owner of the site. It really goes down to you as the owner. Do you want to have a more personal relationship with your users or do you have thousands of users and dont want to necissarily deal with disputes and transactions.