If you need a script that can built such a website for job posting, I'd suggest OSClass. They do have a job posting software, as well as one for more broader Classifieds type of websites.
Without looking, I'd guess you'd be able to find an API to pull jobs from a 3rd party service. I'm pretty sure I saw a site that was pulling elance jobs onto their site.
my firm built guru.com - let me tell you, this site took us 6+ months to develop on a roughly 100k budget. Are their scripts out there you can pickup for a few hundred and customize? Sure. Does that mean you will be a player in the market with elance, guru, freelancer etc. Probably not. Good luck though.