bulletproof wrote:"Is it necessary to have a permit to purchase for private party sales when you know the person is legally able to purchase firearms?"
Not always.
That's strictly true, but somewhat misleading. If you know the person is legally able to purchase firearms, the answer is simply no, a permit is not required.
In fact, a permit is never legally required for a private party deal. The legal requirements to make a private party sale are as LarryFlew posted: The buyer must not be a prohibited person, and it must not be reasonably forseeable that the buyer will be likely to use the firearm in commission of a crime of violence. From a legal standpoint, the permit is not necessary. The parts of the laws which require sales to be reported to the police explicitly do not apply to private party sales (see 624.7132 subdivision 12). If you don't personally know the buyer, you are required to see, though not record or report, government issued proof of identity which includes a photo, name, address, and birth date.
In fact, it is entirely possible that you could be shown a valid permit and have the sale still be illegal. If the buyer's response to "why are you buying this?" is, "there's this guy who's been hitting on my wife and he's pissing me off," then you should not sell him a firearm, even if he shows you a valid permit, as now you have committed a crime if the buyer goes out and shoots someone.
In fact, legally, a permit to purchase is never required at all. The law mandates a waiting period for purchase of pistols or "semiautomatic military style assault weapons", so that the police have time to conduct a background check. The permit to purchase is a convenience for buyers, so they don't have to wait on the background check during every purchase; for dealers, so that they don't have to submit paperwork to the police and wait on a response; and for police, so they don't have to spend so much time checking up on buyers. Whether you can find a dealer who will make sales to persons without a permit is a different matter.
The bottom line is that you, as a private seller, are in violation of Minnesota law if you make a sale and:
1) The buyer is someone you don't know, and you don't verify their identity by viewing their ID such as a passport or driver's license
Or
2) The buyer commits a crime with the purchased weapon within one year of the sale, and at the time of the sale, the buyer was a prohibited person or a reasonable person would have thought it was likely that the buyer would use the firearm as part of a crime.
Viewing a permit as a private seller is a good (but not foolproof) way to cover your legal bases regarding the second part of #2 above.
I'm basing what I've said on the printed statutes. I'm unfamiliar with any applicable case law. If someone knows where I can read up on that, I'd love to hear it.