Hi! I have a Marlin 12 pump gauage Model 21. Can anyone verify what it's worth?

in that condition id say $100 if that

How long is the barrel and does it have any engraving on it?

I don't know what the real world value is, but it is listed in the Standard Catalog of Firearms, the plain Jane, Grade A at 400 in excellent down to 100 as in parts gun.

What Ron said there. The grades are such that the grade A is the plain Jane version of that gun. I'd guess a couple of hundred if it works properly and you find the right buyer.

You could cut the barrel down to 18 - 20" and sell it to a cowboy action shooter, or better let him cut it down.. They need to have a hammer shotgun, either pump or lever, and scour the market for Winchester 1897 pump guns.

Your Marlin is less sought after - Marlins seem alway to be, but it still would make an authentic cowboy/action/reenactor gun for Pecos Bill or someone.

The Cowboy crowd likes the hammer guns. Maybe you could sell it too someone who could fix it up and use it. Have it checked by a Marlin shotgun smith, some guns broke the bolt with modern ammo. Value about 75.00