How much is a vintage Parker Brothers side by side 20 gauge shotgun worth?

It will depend on model number, year of manufacture and condition. Parker is one of the better brands, so it would be worth checking with more than one seller or collector to get an idea of its worth. Always ask at least two sources, to be sure you get a fair estimate.

Keep in mind there were also inexpensive copies of Parker with similar markings such as "T. Parker" and "D. Parker", that are worth very little.