Swapping application in Joomla

This is a simple swapping application. A user can buy and sell items. It will have Pyapal standard for paymanet and Purolator for shipping. Funtionality overview: A user can signup as a free or paid member. They can post an item to sell or buy it. While buying their address will be verified by Purolator api and after selecting their shipping address and preferences Paypal payment will be done. This need to be done in Joomla. HTML is already done. Should be done in 2-3 weeks.