There is not much anyone can do about the existence of mailing lists being sold from one spammer to another. You might be surprised at how many ways there are for someone to get your e-mail address. From posts to newsgroups with your e-mail address. This seems to be the primary way spammers get addresses. From mailing lists.
Spammers regularly attempt to get the lists of subscribers to mailing lists.
From web pages. Spammers have programs which spider through web pages, looking for e-mail addresses. From various web and paper forms. Via an Ident daemon. From a web browser.