Buy and sell webpage/program

The idea is to swap, buy or sell online. Customer must be able to upload to server a description of product and insert picture, price for purpose of selling. Buyer must be able to view, enlarge picture and see specs. Buyer and seller must have a closed communication channel so no others can see. If product is sold it must be flagged or removed from site. Payment should be left to buyer and seller. Let's start with that and see where it takes us.