What I'm putting up is a brief, brief primer on the basic outline of the rules that govern buying and selling guns.  At least at the federal level.

I suppose the first thing to recognize is that the laws and regulations are full of twists and turns that constitute a trap for the unwary, the trap being a felony conviction.  A lot of this has to do with varying definitions in different state laws, vaguenesses or indefinitenesses in statutory language, and this makes for a really complicated mess.   A lawyer in New Jersey specializing in gun cases - that's the vast majority of his busy, busy practice - once published a book called "New Jersey Gun Law".  It was on the order of 2000 pages long.

Twists and turns in state law make for headaches.  A simple .22 "long rifle" cartridge with a hollow point bullet - ideal for hunting squirrels with a .22 rifle - can be perfectly legal and sold across the counter (or even stocked on the shelves of your WalMart where you can help yourself) in one state and expose you to get you 3 years in prison in another.  An empty pistol which is an accurate replica of an old-west-style sixshooter can be perfectly legal in Vermont and get you a felony conviction across the border in New York (if the trigger pull is too light for New York law).  And so on.

So, let's get back to buying and selling.

Let's assume Buyer lives in State A and Seller lives in State B.  Buyer wants to buy a long gun - a rifle or a shotgun - from Seller.

Let's assume, for the sake of ease, that States A and B both have no restrictions on sales beyond the federal laws.

Scenario 1.

If (1) Buyer is physically in State B and (2) can take delivery of the gun "in hand" from Seller, and (3) Seller does not hold an FFL - a Federal Firearms License - then (4) Buyer presents some ID to seller showing Buyer lives in A (where the rules do not go beyond the federal baseline) along with cash (or other mode of payment), Seller gives Buyer the gun, and that's that.  Private sale.

The problem with this is that Seller leaves himself exposed to criminal liability of providing a gun to a person not entitled to have one (by reason of prior felony conviction, existing domestic violence restraining order, etc.) because Seller did not have a background check done.  But, that's not necessarily a problem, particularly if Seller knows Buyer is not such a person.

Scenario 2.

Same players as above in #1, but Buyer is not physically present in B and wants Seller to ship him the gun.  This is the common scenario when connecting with a seller through sites like "gunbroker", "GunsAmerica", "GunsInternational", "AuctionArms" or when looking at the websites (or print advertisements) of some of the bigger (or smaller) gun stores.

Federal law says a long gun transaction has to go through the holder of an FFL located in Buyer's home state of A and Buyer has to have a background check.

In short, the FFL acts as an agent of Seller.

So, Buyer goes to an FFL holder in A - someone convenient to his home, usually - and gets a copy of the FFL's actual license, signed by the FFL.  Buyer then sends the copy of the FFL and payment to Seller.  Usually sellers want money for shipping added to the purchase price - that's a given.  Seller ships the gun directly to the FFL in A and not to Buyer.  If Seller uses the mail, he ships it to the FFL's mailing address.  If he uses UPS or FedEx, he ships it to the FFL's street address.  When packing the gun, Seller is not supposed to have anything on the package which might indicate "there's a gun inside" but he is supposed to tell the shipper "there's a gun in this".  This rule - it's part of the regs - is to prevent someone from stealing it in transit.

When the gun arrives at the FFL in A, the FFL lets Buyer know it's arrived.  The FFL then also puts the gun's information into his ledger book.  The FFL is required to maintain the ledger and keep it for a long period (20 years, IIRC) after he ceases doing business.  ATF and law enforcement can demand to see that ledger any time and the FFL must comply.  Buyer then physically goes over to FFL's business location to pick up the gun.   When Buyer arrives, he gets to look but not take the gun until several things happen.  First, Buyer fills out a 4473, which is an ATF form that asks, under criminal penalty, for identification (including SSN) and a series of questions to determine whether Buyer is a person under a legal disability that would prohibit him from possessing a firearm and also whether Buyer is buying the gun for himself or for someone else.  (If for someone else, that other person has to go through the background check.) Buyer provides ID which shows he resides in A.  The FFL then takes the completed form and ID and some cash for his services (in the $20-$50 range, usually) and goes into a back room, where he telephones the background check number.  The FFL gives the background check people - who do not work 24 hours a day - his own identification and then communicates the information about Buyer to the background check people.  They, in turn, will tell the FFL "Proceed" or one of a couple flavors of "no".

If the background check is "proceed", then the FFL notes his ledger book and gives the gun to Buyer.  

If the background check is not "proceed" the transaction does not go through.  The FFL keeps the gun until either the Buyer tells him to send it back (while trying to get his money back) or the background check can be straightened out such that it can "proceed".  But Buyer goes home empty-handed.

The background check serves to immunize - though not totally - the FFL and Seller against liability if Buyer is not entitled to have a gun.  Though, if the FFL knows that Buyer is lying, he's still exposed.

Scenario 3.

Buyer and Seller both live in A and want to transfer a long gun.  Seller is not a "dealer" or "in the business of buying and selling firearms" - just a private seller of this one gun.

Basically, so long as Seller is satisfied with Buyer's bona fides, Seller can give the gun to Buyer without a background check.  This, of course, leaves Seller exposed to liability if Buyer was lying.  Running the transaction through an FFL and getting a background check is the way to go.

Scenario 4.

Same as #3 above but Seller has a lot of guns and buys and sells guns.

The transaction really should go through the FFL.  This is one of those grey areas - how much buying and selling is enough to trigger a requirement that Seller get his own FFL? - that makes for headaches.

Getting an FFL is not nearly as easy as it used to be and requires a certain level of both investment and government involvement (like police approval) that didn't used to be there.

Scenario 5.

Same as #1 above, but Seller holds an FFL.  In other words, Buyer goes on a trip to B, stops in a gun store, sees one he likes and wants to buy it.

In this case, Seller will need proof Buyer resides in A and will make Buyer comply with A's laws.  Thus, if Buyer is from, say, New York, to buy the gun from Seller he'll have to comply with New York's laws governing how many guns he can buy and how often and provide the documentation of his permits and so on.  The other potential buyer who lives in a state with less-restrictive state laws - like Vermont - will only have to comply with Vermont's laws to buy the same gun from Seller.  Thus, you cannot avoid your home state's laws' requirements by going out of state.

As a practical matter, commercial sellers of guns will require any buyer to go through the background check.  They have a business to protect above and beyond their personal exposure.

You might ask why anyone would want to do a private sale without a background check.  The shortest answer is that they don't trust the government to not use the information derived for some nefarious purpose.  Or, in short, they don't want the government nosing about their business.  They've lived long enough to know that in the history of the world, every time governments have collected information about who has firearms and what the firearms are, it was a prelude to confiscating them and turning that country into a police state.  And they don't trust their government.

Look at it another way:  would you be willing to go through all this to read or write a blog post?

Maybe more later....