Captcha and Craigslist

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I've been checking out a lot of these freelance websites and I always find projects posted wanting people to do captcha work and post ads on Craigslist. I know what a captcha is, but why do they only want teams to do it? Does it require something special that I'm not aware of? And the same question with posting ads on Craigslist, people keep complaining that they can't find a good ad-poster for Craigslist, does it require something special to do these jobs?