Yes, you say absolutey right. I have worked for many clients, only some of them paid for my work, others did not pay. Most of the clients give hourly job and get interviews and change it fixed price job, they take the work of contractor and never pay. So, I reach on a conclusion that in such a way oDesk would become the form of fraudulent persons.
PK wrote in part: "Most of the clients give hourly job and get interviews and change it fixed price job, they take the work of contractor and never pay."
The answer is simple, don't accept the fixed price job offer!
"Form of fraudulent persons...."? In the situation described, oDesk is not responsible. The client changes their mind about whether they want to hire for an hourly versus a fixed price job. Again, the freelancer has the prerogative to decline the offer of the fixed price job.
Also, here are some posts about resources to help when writing in English:
English Writing Skills - Part 1 http://community.odesk.com/t5/Freelancers/English-Writing-Skills/m-p/60114#M21640
English Writing Skills - Part 2 http://community.odesk.com/t5/Freelancers/English-Writing-Skills-Part-2/td-p/60267