It sounds like you are headed in the direction of WordPress. Do you want to create an e-commerce portion, too? Check out http://www.woothemes.com/woocommerce/. Cheers! Let me know if you have any questions.
I had a chance to discuss with the creators of Mayocat Shop a couple weeks ago. Their goal is to build an open source marketplace solution, which sounds a lot like what you're looking for. You can find out more about their solution here: http://www.mayocat.org/
Gun.io is a large marketplace of freelance OSS devs.
I do a search for something like: 99designs clon, Fiverr clon that will usually yield best results.
The answer will typically depend on the type of marketplace you're building (i.e. ecommerce, products, services, etc). For instance, you can go to shopify to build your own product marketplace and there a few others. Let me know if you'd like to discuss this with more specificity.