http://www.rpgfreelancer.com/ rpgFREELANCER.com is a one-stop location for publishers to find freelancers for their projects. Simply post your project and freelancers can make offers to complete it; then select your preferred choice (you don't have to take the lowest bid - whichever one best meets your requirements in terms of price, delivery date, portfolio, feedback, and so on) and wait for the work to arrive. With secure payment facilities, project tracking, feedback for both parties, and more. For freelancers, you can even get emailed when a job is posted which fits your area of expertise!
Although I'm not sure about "bidding" this looks like a good exchange for game writers and artists.