In the United States, a straw purchaser of a firearm at a federally licensed firearm dealership who lies about the identity of the ultimate possessor of the gun can be charged with making false statements on a federal Firearms Transaction Record, which is a felony. Note that in this case, purchasing the item for another person is ipso facto illegal, regardless of that person’s status as a legal possessor.
from Wikipedia
I guess if you’re buying up a lot of guns with the intent to resell/distribute them, that would count as straw purchasing.