That's what I mean - anyone with any sort of skills in multiple languages very quickly 'graduates' from rentacoder, or picks only very well paying projects, because they can, or focuses on just a few clients that they have prior relationships with.
So a buyer coming in with a new request won't have this person available to him, and should he find a person, he will also try to monopolise the persons time, taking him out of the market.
True, but what you said also applies to freelancers not working through RAC or similar websites.
After a year or two of freelancing and if you're good, you'd pretty much get to choose the projects you work on. I usually go with the projects that are interesting to me(research projects from my local university) or that come from good clients or friends.
So a buyer coming in with a new request won't have this person available to him, and should he find a person, he will also try to monopolise the persons time, taking him out of the market.