the shotgun takes a 2 3/4 shell
Not sure about the 12 guage but the 16 takes 2 1/2 to 2 3/4 inch shells probably the same for 12.
You need to provide a DETAILED description of ALL markings, barrel length, caliber, sights, stock material, box, papers, accessories, finish, overall condition, etc..
In "good" condition with standard barrel length (3.25") and with standard Nickel finish - value will range from $125 to $150 US.
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
From the breech.
From the breech
A little over 36"
Any shotgun with a barrel length of less than 18 inches and/or an overall length of less than 26 inches.
A shotgun is a shotgun, regardless of what you do to it (except should its length go below certain limits, it will be reclassified as a pistol or handgun in some jurisdictions). If you saw some of it off, then it becomes a "sawn-off shotgun", pending the above restriction on length.
From the muzzle to the end of the buttstock.
18.5 inches
18"