Nothing illegal about selling silencers, so long as an exchange isn't made before the buyer is approved by the BATF and has paid for their tax stamp. I often times see class 3 guns for sale at gunshows- you won't be able to take it home, but you can start the paperwork or show interest and follow upwith the seller and BATF afterward to make sure all of the proper forms are filled and the proper approvals are received before the gun changes hands.

One nice thing about gun shows is that if you ever get the urge to refinish your old hunting rifle with tru-oil, all you have to do is go to gunshow to see a few dozen other gun abortions performed with tru-oil to cure you of the urge.