The problem with the barters you propose are that SEO is targeted to companies (web site owners) while billboards are targeted at typical consumers, buying holidays or clothing or groceries or electronics or whatever, generally the things you don't need for your business.
How many billboards do you (as a billboard expert) see for things your startup company needs? Then you know what companies to ask.
Kees
"I'm working on a new startup in a DIY fashion in the city of San Francisco, and as I have a limited budget I have an hard time to hire companies or agencies doing the services that I would need. I also have limited time (and sometimes also lacking of sills) to that yourself as I already have a full time job…and that doesn't help at all!
As my service is very innovating and interesting (I sell mini bill-boards advertisements in the areas of San Francisco and Bay Area), I was wondering if there is any site there around that would allow me to barter my service with the other business services; such as I give free advertisements in my billboards and they in exchange give me SEO services, or legal, and so on.
I have searched online and only found the diycity.org anyone that could suggest more sites?"
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic