This will probably not be well-received by many, BUT.......
I, for one, would gladly "purchase" an annual "subscription" for $10 or so to help defray some of the cash expenses of the site. To encourage new members, visitors could sign in a certain number of times before having to subscribe...
Maybe also a token "listing fee" for listing items in the "For Sale" forum?
Just my initial thoughts to maintain a great site,
Byron