The fundamental error with trying to fix such marketplaces via another marketplace is that contractors are typically far better off relying on repeat business and some elementary lead prospecting, while project owners are better off relying on networking and repeat business.
Your target market is thus (more or less) restricted to people who don't yet meet that level of experience and professionalism. You get the contractors that have no existing clients and no clue how to approach companies, and you get the project owners who are out to get bargain-basement labor and firmly believe they'll be the one poster who gets lucky and finds a contractor who works on the internet yet is too inexperienced to realize what that means for their billable rates.
This is still a potential market, but you've lost the more desirable end of the pool before you even leave the gate. And even if that is the end of the pool you want to provide value to - you can provide far more value by helping them move upstream, rather than trying to stamp out another marketplace which is a bad solution from its conception.
Your target market is thus (more or less) restricted to people who don't yet meet that level of experience and professionalism. You get the contractors that have no existing clients and no clue how to approach companies, and you get the project owners who are out to get bargain-basement labor and firmly believe they'll be the one poster who gets lucky and finds a contractor who works on the internet yet is too inexperienced to realize what that means for their billable rates.
This is still a potential market, but you've lost the more desirable end of the pool before you even leave the gate. And even if that is the end of the pool you want to provide value to - you can provide far more value by helping them move upstream, rather than trying to stamp out another marketplace which is a bad solution from its conception.