My local dealer once told me never buy a used gun at a gun show, that is where they take the turds to dump them.
That's a danger encountered in buying anything used, not a specific feature of gun shows. Why would someone have sold his used firearm to a local storefront dealer? Probably for all the reasons people have sold guns at shows.
I know that there are good prices to be had at gun shows, because when I decide that a gun doesn't interest me anymore, I see what it will fetch at a show and if someone comes close to a price that doesn't outrage me, it's gone.
I do also see high prices on lots of items. Occasionally, one sees something unusual. For more than a year back in the 1990s, I had my dealer check with Jerry's Sporting Goods for a Marlin Biathlon rifle. It was in the catalogue every month, and every month they said they were still waiting to get it in. It was $450 from the distributor if memory serves. I found a fellow selling one for $375 with the complete sight set, biathlon rail and four magazines. I leapt on it and shot it twice a week for about a decade, and it printed a three inch group at 100 yards with the iron sights just before I sold it.
If your know your interests and keep your budget in mind, a show is just another opportunity.