Staffing Leads

We crawl a few dozen job boards for a niche job board of our own, and the hardest part was never finding postings, it was deciding which ones are real. The same role shows up on the employer's careers page, on three boards and in two agency reposts with slightly different titles, and some stay live for weeks after the seat is filled. For a recruiter buying leads, an agency repost is basically a competitor's ad. How do you separate the employer's own opening from agency reposts, and do you drop roles that have gone stale?