Please read the description thoroughly and send me a message with how you plan to do this project.
I'm looking to create a simple transaction system for demonstration purposes (no need for real money transactions).
People can register as a User (buyer) or Merchant (seller).
Merchants can sell products on their own, third party, website or app by using the API and a simple callback script on their server. Buyers can buy the product on that website. Upon clicking a 'Buy' button on the merchant site, the user gets taken to a purchase page where they have to log in and confirm the transaction with a receipt (basically like PayPal). For security purposes, merchants have to request a secret 'key' for every website they want to use the transaction system with (so every website can be tracked for performance).
The project is relatively simple as there's several open-source systems (OpenTransact, PaySwarm, OAuth) that allow you to easily integrate this.
If you're interested, send me a PMB explaining how how you plan to make this.