Swap n Shop was the previous section for buying, selling and trading. It was replaced mid-year this year with the For Sale/Trade system, which is a little more intuitive but does not allow comments, etc. like the Swap n Shop did. You cannot post in the Swap n Shop anymore, so your only place now to buy, sell, and trade is the For Sale/Trade section.
"Simplicity is the ultimate sophistication." -- Steve Jobs