Since the Parker guns have been out of production for over 50 years, I would start with the Parker Collector's Association.
Your Parker Bros. shotgun was made in 1925.
A good place to try is gunbroker.com
It would top out at $250 as a mantle decoration. Unless you mean a Parker Brothers shotgun. Then it could be several thousand dollars.
To determine the exact value of a Parker Brothers 12 gauge double barrel shotgun manufactured in 1878, there are a few factors that would need to be taken into consideration. Most important of these factors being, the condition of the shotgun.
I have seen one listed for $1750.00
No. What would the company that produced the finest American double barrel shotgun want with a utilitarian import?
Very late in the 1800's
Under trigger
The value of a 28 gauge Parker Brothers shotgun can vary significantly based on factors such as its condition, rarity, and specific model. Generally, prices can range from a few thousand dollars to upwards of $50,000 or more for exceptional examples. Models like the Parker AHE or other highly sought-after variations tend to command higher prices. For an accurate appraisal, it's best to consult with a firearms expert or appraiser familiar with collectible shotguns.
Best left to a gunsmith
50-500 usd
Any good gunsmith can find or make parts for your shotgun. It might take several weeks to a year or more. Parker Brothers are a top of the line shotgun, and a shotgun in working shape is worth more than a broken one. Do some checking around at a local gun show, or surf the web for parts.