Developing points-based ecommerce
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I intend to develop a points based barter system on php, that allows users to buy products using points/coin that are provided to them by the system. lets say, user A got some 500 sign up points, he wants to buy user B's product that is of 400 points, so he pays his 400 points to the user B and gets the product. user B that had his 500 sign up points now has 900 points as he gave his product to user A. Similarly, user A has the product and remaining 100 points.

i have worked on mysql, html and php. All i am looking for is a starting point for developing this particular point based transaction system, i will have no problem in design the db model, using cakephp modules for login and all other stuff. it is this particular transaction system that i need help with, i am looking for a ready made open source solution/module for sucha system if it exists. Even if you could tell me what term is generally used to regard this sort of transaction system it would be a great help.
Thanks 
