Being from Illinois myself, I've been to a hundred gun shows and set up alot there too. When I set up, I sold reloading supplies. If your just walking around, I would say thats fine. At the shows I go to (Kankakee, St. Charles, Wheaton) people walk around with signs advertising their gun(s) they have for sale. Some folks carry their gun around with them, and some put it in a case and wait for people to ask.
I picked up a revolver from a guy that I'd put money down on, and it was in a case. Nobody questioned me, but I did get alot of "What's in the case, buddy?" and then they would ask if it was for sale/to see it, but not about its legality whatsoever.
I would say yes, you can sell stuff out of a backpack just walking around. If you really have mixed feelings, call the show organizer and ask.