The main risk that such a system will expose you to has nothing to do with programming, but is worth mentioning anyway: operating such a system will cause your web site to be classified by many governments as a money transmitter, which will impose a number of complex legal requirements on you. It will also make it very difficult for you to move money in and out of the system, as most of the normal providers (e.g, credit card processors and PayPal) do not permit the operation of money transmitters.
Consult a lawyer for details. This stuff is very complicated, and the details will depend on what countries you intend to do business in.