if someone bought a firearm from a gunshop and then stepped out the door and sold it to someone else, he may very well have a serious legal problem regarding a 'straw purchase'.
Actually, that's not a straw purchase. He bought it and resold it.
Let's say you're the guy buying and I'm the guy in the parking lot. If I give you the money to buy the gun on my behalf, and you lie on question 11a, then that's a straw purchase. The essential component is the fact that it's a proxy buy.
While the behavior described in your post could certainly risk serious scrutiny, it would not be a straw purchase under the law. Even if it was only for 30 seconds, you were the actual buyer.