I have some experience using Guru.com. As an artist, you can make money, but you won't make much. The business model favors employers and attracts low paying employers for the most part. I don't use the site anymore, but it seemed like I always had problems with employers when I did. A lot of them tried to get me to do free work too.
Most well paying employers don't use freelance market websites anyway. They use artist directories, find artists through various connections or artists contact them directly. They have larger budgets, so they don't need the lowest bidder. They hire who they want.
So freelance market sites can be either good or bad, depending on who you are and what you want. I'm not going to say anyone is right or wrong for using them, but you should know what you are getting into.
BTW, I don't think it's fair to compare freelance market sites to eBay. One business model drives the price up for those providing services, while the other drives the price down.