It's against the law to sell a firearm across state lines without involving an ffl.
This seems to go against the part in the 2nd amendment "...the right of the people to keep and bear arms shall not be infringed." Particularly the infringing by having to involve the Feds in a private sale.
So yes it is against the law to sell firearms to someone from a different state. Is it right on the other hand is another story.