Hmm, I don't know if I'd be selling my email list. It might be ok if only the intended party uses it, but who's to say who you sell it to won't sell it to someone else, and etc etc.
I would keep it under your 100% control if I were you. For instance, you can send mass emails to all of your members if you want, and the user hasn't opted out, correct?
If this is the case, how about you tell your advertisers that you can send out monthly newsletters, and they can buy space in each newsletter, advertising their product, service, etc.
This way, you are in complete control of your member's addresses, you are in complete control of the content in the e-mail, and by ensuring it comes from you, your users will likely not consider it spam.
Just my two cents.