It's easy to set up something to collect the funds. Authorize.net can do that. The tough part is distributing the money back out. You can probably set up something through your bank that will give you an API for distributing money to a relatively limited number of people - like maybe up to a few hundred - but doing it dynamically to large numbers of people is much more difficult. What you could maybe do, assuming the people all live in the US, is just mail them a check if they request it and their balance is over a certain amount, or manually PayPal them. Direct money transfers would be reserved for whoever is doing the most business through you - sort of like a perk.
Go talk to your bank about ACH.
