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I've worked on a job board site before, but it really didn't go anywhere because there was no real information value other then the non-unique job listings.
The site used a feed to pull in new job information.
To make it work from a rankings point of view, my thoughts are to make the main category pages very strong information wise, resources, etc
That will help to acquire backlinks to these pages and get rankings for specific job titles.
This seems to be a popular solution these days to start a job board, I've not used it though.