At ThemeForest, consumers can shop for quality themes for such platforms as WordPress, Drupal and others while developers can find a marketplace to sell their themes.
After creating an account you deposit cash into the system and can then purchase and download themes. There are themes geared towards personal promotion, whimsical and business and they start at $10 each. Users have profiles, as do developers. This should provide a nice way to get support for those themes should you have any difficulties.
Developers, or Authors, are paid on a tiered system - starting at 25% of every sale. When $500 worth of files are sold you are given an "exclusive" status, whereby your themes can be exclusive to ThemeForest and the payment rises to 35%, up to 50%. Market your files on your own and you can raise that number to 75, even 100%.
If you are a member of the site, there is also a referral program - using cookies, if a user signs up through your referral and deposits cash, you get 50% from their first deposit.