Specifically on sites like freelancer and fiverr, it seems as if literally nothing is stopping a client from:
Sending me requirements
Waiting until I deliver the work (or insisting that I deliver a working copy and not just screenshots for payment)
Telling the "system" that they are not satisfied, effectively getting their money
It is notable that the person has to have the funds already - but nothing stops them from retracting the funds. Its not like I get a say in the matter, if a customer says that the work isn't satisfactory, even when an idiot could see that it was. This actually happened to me and now I am scared that every job I do will be community service. As such I stopped applying/answering on these types of sites.
Looking at the successful people on these sites, I wonder how they got any money at all. How in the world are these sites a viable source of income if it basically is set up in a way where the client can choose to pay or not regardless of the delivery?