I believe this is not something you can achieved without the help of a Payment Gateway through API e.g. paypal, echeck.net, evp snap and many others.
In creating this type of application, the first thing that should come up for consideration is "Liability". It is always nice to have a middle gateway or company who can absorb any liabilities incurred by either party just in case..
Actually they are not hard to find. Here is one good example . The integration can be found here.
If you want to use paypal, you can use this. The example scripts are already included. I tested this on my paypal sandbox 10 months ago, and I can pretty much confirm that it is working.
Before processing real payments, make sure to test your scripts in the sandbox. Paypal have an excellent sandbox for you to practice. In fact, you can create both the seller and the client accounts in the sandbox.