Can you help me about an Open Source System?

Hello

I am looking for an open souce system to be used in e-commerce, but the
clients-visitors don't buy products, they will buy services.

For example, imagine a doctor especialist in sex. The people could make his
questions in internet, and pay it with the Visa credit card. 24-48 hours
after this, the client will receiv the answer of the doctor.

Can you help me? do you know any system like this??

Thank you very much at all.