Here in Illinois, if you are at a gunshow, you better have your FOID card in hand before you even ask to look at a gun.
That alone scares off a bunch of thugs, gang bangers, felons, etc.
So, to answer your question, no I have never seen anything that looks like a blatant straw purchase.
On the other hand, prior to Christmas, I might have seen parents buying guns for their kids, or a spouse buying or shopping for a particular gun, and I might recall a time or two where said parent or spouse called to whomever to ask again what little Johnny wanted for his first .22 rifle.
Definitely not the gangbanger demographic, and like I said the FOID card acts like a pre-screening process....NOT that I am recommending for you guys who live in free-er states, however.
It didn't make my spidey sense get all tingley.
When I was stationed in North Carolina, I did have one guy standing next to me at a table pick up a shiney Taurus PT 92, then asked the guy behind the table if he had a brand new one still in the box, never been touched...he turned to me and said something like "I don't like other people touching my guns." The vendor guy informed him that they didn't have any new unopened ones. Before the guy walked away he mumbled something to me and the only parts I made out were, ".....Jews mumble mumble world mumble mumble domination mumble mumble."
Yes, I do enjoy the people watching at gunshows too.
I like it when the one guy thinks he has the totally hot girlfriend/wife and has to walk around with her for hours as you can tell she is obviously bored out of her skull, just so, well, I guess, he can puff up his ego as all the men in the room ogle at his woman.