There's been a lot of controversy over WordPress theme sponsorhip lately.
In response, I've just done an extensive and detailed write up on how to evaluate Wordpress theme sponsorship. Pick the right theme and you could get up to 500,000 almost free backlinks with anchor text.
Unfortunately, most theme sponsorship doesn't end in thousands of backlinks.
I cover both how to choose a good candidate for sponsorship and how to conduct an audit after the fact or on the designer's other work.
Don't believe themes.wordpress.com download figures. Downloads are next to worthless to the buyer. Long term implementation is what you are looking for.