A mailing list I belong to did something along these line some time ago, it may work here.
Make the site a voulentary pay site. Allow everyone to post, but reserve special privleges for those who support the site. Some ideas may include:
A special designation for supporters, such as supporting member, or charter member for the first to sign up.
Allow posting a picture by the name like Goatboys.
Limit non-paying memebers to 5 or so for sale posts per month, if you are selling that much stuff you can support the site.
Limit some archieves to paid members.
Limit smileys to paid members.
Give supporters firstname.lastname@example.org emails, fowarded to thier regular email.
Have some other incentives.