Some people are passionate to work on the things they like. For example, I like to work on Websites and if I had to solve few bugs on a site, it would not bother me that much. I can even do it for free. Haven't you seen people on StackOverflow, Quora, Reddit etc.? People helping others and reviewing codes? What incentive do they get? At least on Taskbarter, they'll get task points that they can redeem on hiring someone.