Federal law regulates sales by federally licensed gun dealers (FFLs) (defined as a person “engaged in the business” of dealing in firearms), but does not apply to a non-dealer, (defined as a person “who makes occasional sales, exchanges or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or a part of his personal collection of firearms”). 

Sales by non-dealers in Maine take place at gun shows that are a significant distribution channel for all firearms and they are advertised and sold extensively in Uncle Henry’s. Presumably sales also take place in a wide variety of other forms and venues, although Craigslist and eBay expressly prohibit advertisements or offers of firearms.

State law does not regulate sales by private sellers at gun shows, catalog sales via Uncle Henry's or otherwise, except that no handgun may be sold to a person under age 18, or long guns to those under 16. The minimum age in Massachusetts to purchase a handgun is 21 and a long gun is 18.