Parker 20/28 gauge

[Mills]

Very interesting gun. It is hard, if not impossible, to judge value by photos on the internet alone. There are a bunch of PGCA members in the Massachusetts, New England area who would help you. If you have some documentation to go along with the gun including old photos and the like, it should increase the value too

[John Davis]

Who would have thought the 22 inch barrels were original? Nice gun.

[edgarspencer]

Particularly nice gun, despite the evidence of the 20ga barrels being cut. I would argue that the documented length of the 28ga barrels is a large bonus.
Interesting that the butt plate is from the Remington period.
I would caution you to get more than one appraisal before setting a price to sell your gun.