I disagree (although perhaps I'm one of the shoddy developers bidding low to provide total crap :)
I think that the online sites are a great place to get started if you have nothing else. Do a few good quality jobs and you've got a network of contacts who would prefer your good work to the other scam-artists they've worked with from the site. Also, a number of the buyers are savvy to the problems with low bidders and choose realistic people who bid five times the low bid.
I wouldn't recommend doing that forever, but as I said, it's a wonderful place to start, and get some experience and contacts. The most important part is to *not* be one of the shoddy scam artists - if you made a mistake and bid low (as I've did more than once at the beginning), see it through anyway. Your client may be your biggest source of work in the future.