Reverse auction type bidding system

Hi Coders, We are planning to develop a freelance jobs marketplace for local contractors in Africa. We are looking to purchase freelance website script written in PHP/MySQL that is highly customizable, suitable for high traffic and open source code. We should be free to customize the script according to our need and redistribute (even commercially!) without being violating the rights of anyone. More details about our requirements will be discussed via PM. Looking forward your proposals! Thanks