It is basically trading items of the same price. We did this recently on another team I am on with $5 items. It was a lot of fun and brought attention to all our shops.
I had 13 sales (10 trades) in two days, 3 of which were not bsp sales.
Another team is about to start their bsp and theirs will be $5 $10 $15 or $20 trades.
If anyone is interested i will post more info.