The problem for the seller is that the quick turn around time will be used as evidence of intent. Now other factors such as does he have a relationship with the buyer come into play. If you buy a gun and then 30 seconds later sell it to your good friend it appears as if that was your intent from the outset.
Proving what was going in in someone's mind is most commonly done through their actions. If you point a gun at someone and pull the trigger it certainly appears that you intended to kill them.