You should be an experienced PHP and and MySQL coder.
It is recommended for the coder to visit the current Alphabays Marketplace to get a look and feel to the recommended functionality.
There should be a possibility for a user to register either as a vendor (sellers account), or a buyer (normal account). A vendor should be able to upload pictures and info to his items.
There should be an Escrow functionality. The site should get an adjustable percentage of the finished deals.
There should be an admin panel for maintenance, resolving disputes and support.
There should be a Bitcoin handling mechanism. It is recommended to run Bitcoind as client only to connect to blockchain.info. No blockchain download, no local wallet. 
The suggested achitecture is a LAMP-stack, but other sort of arrangement could be possible as long it runs under Linux.
The forum part will be a ready made forum script and not required to be coded.
26 freelancers are bidding on average $4254 for this job
Hello Very interested in your project I can do it with 7+ years of PHP, MYSQL programing experience Review my work on my profile Let's have a talk for more detail Thanks. :)