You can have two types of job postings - One is fixed bid and other is hourly. If you selected hourly then contractors demand 30% down before they start working for you. Once they get that 30% then they are gone and Odesk does not care to help you to get your money back.
And, in case you decided to go with hourly then people keep on asking for more hours and you can't see the work until they have clocked few hours. You still have to pay even if you don't like their work. Again, oDesk will not help you to get your money back.
