How would they know?
Here in SC when you go to a gun show folks walk around with signs on their backs advertising whatever they brought with them to sell and tables are set up for private sales of guns. The result they are looking for is to make it criminal to have those private sale tables and to bring your own guns to sell by just hanging the rifle on your shoulder with a price tag or even for handguns.
Currently we are allowed to sell or trade guns privately and a gun show is a good place to market them without the instrusion of the government. If this passes we will have to worry about undercover federal cops trying to entrap us.