Yeah I agree with you both - there are some real scum bags out there, who essentially will try to take advantage of us, which goes beyond these job bidding sites.
One of my friends is a graphic artist who freelances for small business, and even though she goes through traditional channels to get work, occasionally she will get bad clients who do the same thing: e.g. agree to work being done, and then at the end the clients say they don't want it anymore for various reasons.
For that kind of situation, I think it's a must to have a contract signed, with stipulations on debt collection and legal pursuit.
Even then it's not guaranteed. We had a client who we did work for, which was completely successful, but then she turned around a month after the payment due date and abruptly said she was dissatisfied with our work.
Now this was *with* a formalized contract signed. I tried working things out with her, even giving her the benefit of the doubt by offering a discount just to resolve things, but I gradually realised, she was basically out to scam us. At that point I was pretty pissed and decided to take her to court. It's been more than a year now, and after numerous legal proceedings, we still haven't got our final payment. We're getting much closer, but even with legal checkpoints in place people can still get away with murder.
So sorry about the ramble - my point I guess is that we need to put mechanisms in place to protect ourselves in case the above happens. If possible cover yourself off with a contract (might not be applicable internationally).
The other thing I can think of is that with Freelancer.com, I read that you can use payment milestones for partial work which might make things easier also.