Personally, I think you are thinking too small. IMO there is a larger opportunity for a marketplace to buy and sell promotion for the long-tail of the web (niche content providers and small startups selling to a niche audience) across many mediums including newsletters but also company blogs, social media presences, forums, etc. Buysellads.com does it well, but I think there is room for another player who can execute in slightly different ways.

I actually have been thinking about this idea for a while and would love to chat more about it. I've been working on a travel industry startup for 2 years that has had a difficult time finding great/targeted advertising opportunities.