Question about freelance sites

I have 1 simple question, mainly about elance,odesk and freelancer.

Is it possible for someone to create job offer, get job done from some freelancer, and not pay him? I am looking to get into freelancer sites, and I am interested if it possible on some of these sites to simply not get paid for work you have done?

It's possible but doesn't happen often if you do things right. Each of the job providers can deny your work, which might need to be resolved by Elance itself. In terms of Elance, there are two types of jobs: Fixed and Hourly. With Fixed jobs, you need to send request to fund your project before you start working. This will make sure that your job is funded. Once you finish job, you request release of the funds so that you get paid. In terms of hourly jobs, you track your time with Elance tracker and submit time sheets of your work at the end of each week. Your employer then pays you and if he doesn't, you will get paid automatically after few days.

Similar thing goes with Odesk, but I believe they don't offer protection for hourly jobs. I am not familiar with Freelancer but most of them use same mechanism.