What is the value of a sterling silver tray that is manufactured by Polk Silver Company Model number 3210?

First and most important is the Word "STERLING" also stamped on that tray? If not, it is silver plated and would have zero value unfortunately.

If it IS marked sterling, then you will need to weigh it. Once you have the weight, you can go to a sterling silver specialist like the Silver and Gold Exchange. If you decide to sell to them, they will pay for the shipping and insurance.

Good Luck!