Bro, it sounds like shows are new to you. Before you decide to go set up a table, you want to take in a show or two as a spectator. Then find yourself a friendly local dealer at one of those shows, and volunteer as free labor to help at a future show, to give you a learning experience.
Things you may have not considered as yet- How you gonna do a background check from inside the show? When you are regretting having that Dr. Pepper Big Gulp, who watches your table while you do the 100 yd dash to the potty- WAY over there? Got a reference book to check prices on the gun that customer wants to trade you? Got business cards for followup business? Want a hot dog? If you buy a gun, is it legal in YOUR area to immediately put it up for resale?
c3 is right on the money...again. Damn, fella, you're good.
Shows vary in vendor fees from show to show. You need to consider the other factors like accepting credit cards and how to do it portably, bringing enough seed money to make change, having a good cellular signal to do NICS checks and how you'll handle running your phone battery down and having a decent enough phone in the first place, whether you will do trades and how to offer what you'll give for trades, and the rest. Basically, you'll have to prepare a mobile shop for the shows and have everything to conduct business there. Not every show venue has the option to have electricity at your table, so you'll have to prepare for this in advance. Even the basic question of taking breaks from the table needs to be addressed in advance. There are vendors that do shows that do not accept plastic and ones that just go to sell whatever they can out of the shop. What you'll have to decide in advance is how seriously you want to sell at shows and what you'll be willing to do to prepare to do it correctly. Your expenses will be deductable as costs of doing business tax wise, but you don't want to be upside down at the end of the weekend. That just isn't good business. The trick is to keep costs down at all times. Overhead is the nemesis of all business. You want to walk away at the end of the event in the black.
There's one point that you have to remember and get comfortable with from the start. You will not make any profits until you recoup your expenses first. I could never get this across to my wife's stepfather when he did another of his business schemes. He would spend money on stuff and not take his gas and time into account, but tell me how much he made profit wise when he was still as upside down as one could get. Drove me crazy! Every dime you spend vendoring is an expense. You have to make those back first, then the profit.
Another thing to consider on the topic of trade ins and selling used. The books of value are all well and good, but there's one primary rule to selling used and antiques that one must consider. Nothing is worth any more than someone is willing to pay for it!! I've seen items that the books value at X$, but the merchant stayed married to the item because nobody was willing to pay X$ for it. Guns are no exception to the rule. I dabbled in antiques when my ex-in laws had an antique shop, I bought and sold classic car parts, and now am an FFL. I know of what I speak. You need to learn the market as an FFL. To be successful at shows, this is important also.