Not sure about the Feds part but I do know PayPal is not your friend when buying stuff like this! There are several gun auctions sites and if PayPal finds out you are buying guns with PayPal they will lock down your account! Most of if not all of the gun auctions sites will not even offer PayPal because of this.
If nothing happens this time, consider yourself lucky and then next time use another form of payment.
Super-D