NavyLT is right. It's a common misconception that a straw purchase means the recipient must be prohibited from owning the gun. Even if they could have bought the gun themselves, it's still a straw purchase.
The actual crime is lying in answer to the question "Are you the actual buyer of this firearm?" on the federal form.
If you answered "yes" to that truthfully, you have nothing to worry about regarding straw purchase.
It seems as though at gun shows the sellers only responsibility is to not sell to someone they KNOW cannot own a firearm, but it is not their job to find out
If they're a private seller, of course. In which case is has nothing to do with the fact that it's at a gun show. The law at gun shows is no different that in any other situation.