For being one guy, I sold quite a few coins in the hallways, and made a healthy convenience premium over spot price doing so.  It was like $9 BTC at the time, and I was asking $20 for one coin or $30 for two coins.  NOwadays buyers are willing to pay more than that just to order them online, so I can only imagine how well somebody would do selling them in person at a booth at DefCon this year.