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I wouldn't say they are as much scams, as much as people expecting something for nothing.
Freelancer is notorious for it, I used to use it along with Odesk (think they are now both the same thing) and you have to remember it's a worldwide platform. Many of the clients are based in south east asia or west africa and the local wages are incredibly low. So they ask for a 500 word article for $1 or something ridiculous.
It can take a long time to wade through the dross and find a decent client. Other sites are better in that they attract mainly clients from North America, Western Europe, Australia - clients who want a good job, not a cheap job. Having said that I never bid for new work any more as I have several clients who come back to me time again. Getting started is tough and you might have to accept a few jobs at a lower rate to get some feedback.
Gave them a go and a lot of the advert are people overseas trying to scam you lol anyone had any genuine work offers