How to monetize 8000 emails

Have a couple sites, one has about 7500 sign ups and the other is about 500 -- problem is they are not double optin, nor was there any notice on them as to the use of the emails. Is there any way to use them safely and cheaply? They are targeted to one category. I have never tried email before so if someone can be kind enough to point me in the right direction I would appreciate it very much!