Source: I founded www.buyads.com and www.isocket.com, which powers the direct ad sales for the web's best sites (like TechCrunch, AOL, Microsoft, etc) We hand invited the publishers that join our market because, as your question suggests, there is some threshold between who should and should not...
Coming from the buy-side, I'm sure I have a different spin than those sales & marketing gurus who I'm sure will be keen to help you & comment shortly. You're lucky because there are SO many avenues these days. However it is hard to keep on top of them all and there's no one stop shop currently....
CJ.com or Linkshare.com. If not there check Viglink.com