As a flautist myself, I've browsed your site often (dreaming, mainly), and a possible solution to your 'spam' problem may be found in simply improving the whole shebang - categorisation of the DB immediately springs to mind (piccolos, wooden flutes, feet, flutes wanted etc.) , rather than a single page of 500 adverts (and a page like that full of 'real' mail addresses is a spambot's delight - an 'I'm interested' button could solve that).

If the 'place an advert' process were a little more 'specialist' - dropdown categories, price ranges etc., especially when combined with some of the ideas above, then your spammers may well not bother. I'd be glad to help with this as a side project if you liked - it's something I've meaning to contact you about for a while anyway, by happy coincidence :) Msg me for contact details.

Cheers,
Ben.