While buying and selling firearms is legal in Massachusetts, it is regulated, and it's very important to know how the laws work so that you do not accidentally commit weapons crimes without knowing that you've made a mistake. Firearms cannot be transferred from one person to the next as easily as most other goods.
For dealers, an FFL license has to be purchased. The Bureau of Alcohol, Tobacco, Firearms & Explosives gives these out. While it is possible for anyone to apply for this, even without a store, it's hard for a private citizen to get. Most people go to FFL dealers when buying guns, and gun shops and sporting goods stores hold these licenses.
Now, you can buy and sell guns if you're just a private citizen. If you don't sell more than four of them in any given year, you don't even need to have a license. The proper paperwork still has to be used, but you don't have to qualify as a dealer if you just want to sell an old hunting rifle to a friend.
Once you break that four-gun threshold, though, you do need to have more advanced paperwork. It is at this point that the government decides that you're not just selling guns you don't need or want anymore; you're now buying and selling them to make a profit. For that, you must be licensed just like a standard dealer.
Before you sell a weapon or buy a new one, be sure that you know if it's legal for you to do so, if you're allowed to own that specific type of weapon and that all laws are followed. When laws are broken for any reason, you also need to know your legal rights.
Source: Smart Gun Laws, "Dealer Regulations in Massachusetts," accessed Sep. 16, 2015