The problem with this and this is also a problem with most of your competitors is that it will end up as a lemon market. Which means a marketplace where no one knows how good the quality is, so the quality just drops and the prices drops. Like very old used cars for example.

You will get a listing, 20 spam answer claiming they'll build a instagram clone for $100 or whatever, and the professional (as in good) devs will run a mile as it's just a waste of their time even bidding. Customers will get crap.

Ironically the only way you can be successful in the sense of keeping the quality and welcoming aspects of the site, is by keeping it small and making it more about your network and connections you trust rather than the general internet.

Graphic design might be the exception. I am mostly talking about coding tasks here where no one can really judge by looking at something someone did (if indeed they really did what they said is in their portfolio!)