Which Software Takes Orders and Makes Payments?

Someone asked me to build a website that can process orders in an "in-house" fashion. Which software platform can accept orders from clients, and then allow other members to process the order and get paid for it through the same site?

I think of ConstantContent, for example... wherein, you sign up as a member and choose either to "write" and get paid for it... or to "buy" an article which one of the writers there will create for them.

Possibly, another example would be "Fiverr.com," where you sign up as a member... then you either choose to be a "Buyer" or a "Seller." Thus, you can see every available order request and choose to buy it. Or, people can pay YOU to process THEIR order.

All is done automatically. Fiverr never even has to contact anyone. You see something there, you order it from the associated Seller, and he processes your order. He gets paid automatically, and you receive your order delivered online -- everything is smooth and almost totally automated.

My search for this solution starts now. Even though I am not totally certain of what to ask for or how to phrase it, we all see this process on sites daily. It is very common, yet I have seen no plug-in or software mentioned to handle such a task for myself.

Anyone know the answers to this one?