The internet has changed many things in our lives (or at least for those of us who use it to its fullest potential). The internet has done two major things for and to collectors (no matter what they collect).
First, it allows the collector to more easily find (or buy, sell or trade) whatever he/she collects. In this case we're concerned with collecting guns. There are auctions where guns are bought and sold daily. I will provide some links later. If you are looking for a hard to find gun with very specific characteristics, chances are that sooner or later you will find it on the internet.
Secondly, it's easier than ever before to find out what something is worth. This has an upside and downside for collectors. But really, in fairness to all, a healthy market benefits from well informed buyers and sellers. Nothing can rile me faster than seeing a knowledgable gun collector or dealer running down the value of a gun to an unknowing owner, in an effort to buy the gun for pennies on the dollar.
The internet practically insures that you can find the gun you want to buy, but there may be fewer "bargains" to be found. This is because everyone can see what others are selling for. The internet also practically insures that you can find a buyer for the gun you want to sell, but there may be more competition from other sellers that may hold down the price you can get. In the old days, if you were the only person in your geographic area with a particular gun for sale, you might get a higher price because the buyer may have to go to great expense just to locate another one. Not so today!
There are two major gun sites that I monitor for guns that I want to add to my small collection; gunbroker.com (auction) and gunsamerica.com (non-auction). In order to buy a gun on one of these sites, you will have to register with the site (basically just create a user name and password).
Once you purchase a gun on the internet, you will need to have a local gun dealer that holds a valid Federal Firearms License (FFL) send a copy of his license to the seller. The seller or his licensed FFL dealer will ship your gun to the local FFL and you will complete the paperwork there when you pick up your gun.
Never pay with PayPal (they have a strict policy against buying guns). I am speaking from personal experience. I inadvertently did this years ago. It took a very long time to get my money refunded from PayPal. Use a credit card to pay when you can, but watch out for sellers that charge a 3% premium for using a credit card. Many sellers say that they are offering a 3% discount for cash, which cannot be earned when using a credit card (BS). Your credit card may provide some extra protection for you as the buyer, of course the extra risk that goes along with giving out your credit card information exists. If you pay with a bank cashiers' check or postal money order, if something should go wrong, the process or procedure may be involved.
If buying a gun from a seller on an auction, look at the "feedback" of the seller. This should give you a good indication of what kind of person or business you will be dealing with. Communicate with the seller as soon as possible after the auction ends. If using email, remember that email can be unreliable due to spam filtering software, etc., so follow up with a telephone call when possible. Be sure to either inspect your gun as soon as possible or have your FFL person inspect it. Most auctions require sellers to give a 3 day inspection period at a minimum. Guns can be damaged in transit, especially long guns, depending on how well they are packaged. And finally, be sure to leave feedback for the seller and ask the seller to do the same for you. The feedback record will act as a future reference should you decide to sell a gun or buy another one. Some sellers will only accept bids from potential buyers that have a minimum number of positive feedback.
I bought this Colt Mustang online in 2001
If you hold a C&R License, as I do, from the Bureau of Alcohol, Tobacco, Firearms and Explosives (officially ATF), there are many older guns that can be shipped direct to you. Otherwise, you will need to have a local gun dealer act as "intermediary" for you. You will be surprised at the very long list of guns that qualify as Curios and Relics.