Developing php point-based transaction system
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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 am a beginner php developer, and to give an idea of my knowledge, let me tell you that i dont even know what term is used to regard this point based system. Since i am starting at level 0 any help would be a boon. I have had some experience with php development but havent developed something substantial and this i intend to be my first substantial development.

I would love to be redirected to some open source module that accomodates my requirements as it will make the development faster. If not, i would really appreciate any guidelines, advice, suggestion or anything that is helpful.

thanks :)