is the final result going to be some sort of product or service that you would be "selling" to people? if so then go to services board and post what skills you are looking for and hire developers to develop what you want. find an escrow and put their payments with that escrow until they finish the job and the escrow releases their payment.
The difficulty here lies in how to define the escrow? Is it a guarantee to deliver the code? How to verify that it is safe? Without any backdoors? How to develop the project in the future?.
I do not think that relying on anonymous people to build an idea or develop a site/app will be feasible in the long term.
usually when someone does something like this, they themselves have coding experience. the problem is that they either don't have the time to develop things themselves or not enough experience to do it right. so they hire someone to do it better (after checking their resume,...) and then only check the code afterwards to make sure it is safe, etc.
as for escrow, there are obviously complications involved but there is no bette ralternative. trust should work both ways.