I also agree with Ten Writer, but something a whole lot of buyers overlook is that the better freelance sites allow their contributors to upload a portfolio of finished work or to link to one. Additionally, buyers should ask to see finished portfolio work before they agree to hire the freelancer. The fault of the freelance sites is that they don't make this a requirement. Instead, they often expect the freelancer to do comps, which, in writing is ridiculous. Once it's in the buyer's hands it's good as gone, whether or not he accepts the bid.
Going a bit off topic now, but you guys got me started! Another thing freelance sites like to do is put the freelance writer in an "employees" shoes or make submitted work a work-for-hire default. IMO, that's totally contrary to what freelance should be. If you're a freelancer, you are running a business and you should be able to set your rates, prices, and policies which are not necessarily those of the freelance site. Ultimately, freelance sites expect writers (and others) to be order takers instead of creative and skilled providers. A big part of good freelance is the ability to consult with a client to help him or her understand what makes good web content and what doesn't.