If you did not sell anything then you need to change something. The problem could be that you are at the wrong show. Last year at Iowa Gas I pulled in with a trailer load of pumps and sold out within an hour. And no one tried to low ball my asking prices. Shows are a great place to meet other people that share the same interest and to sell things your ready to pass on. If you want something to sell it needs to be priced at what you would be willing to pay for it if you were on the other side of the table.
This is the rule to follow. Not saying to give stuff away but if you have a Texaco pump plate and hope to get $300 for it at a show because you seen one sell on ebay you might as well forget about getting that for it at a show. Keep in mind most and i mean most guys at the shows are also collectors and have a good idea of what something is worth. Your not going to get something super cheap but it's just another avenue to add items and get to hold them in your hands.
Guys who price stuff reasonable sell items. Guys who you see at the shows with the same stuff for 8 years straight are either setting up for fun or hoping for some millionaire to come threw. LOL
Last thing and the most important thing to add.... shows are about seeing and talking with gas friends as much as finding a item you need. If your only going to buy something so you can sell it for a profit then your going for the wrong reasons and the shows will not be fun for you. The hobby for me is adding something to my collection and talking with guys that i now call friends.