I didn't read where it said he stole the guns. If stolen then obviously possession of stolen firearms.
How are "his" firearms stolen?
"State Police said Sunday that 53-year-old Stephen Buckman was arrested and his firearms, ammunition and assorted pills were seized."
I suppose it depends on how you view buying fenced goods. Are you a bonafide owner if you buy stolen items, as long as you paid some amount of money? Based on the obliterated serial numbers and the fact he had no "legal" way of purchasing, the chances are the guns were stolen at some point and sold on the black market, ending up with this guy. But that is just circumstantial--no way of proving to a criminal law standard, thus no charges for that.