If they are P mintmarks, they can only be plated. They didn't make Silver Quarters at Philadelphia, only San Francisco.
So they aren't silver and are likely silver plated, or possibly chromed. and definitely won't be worth the effort to cut them out of there.
You can list it as a BIN and best offer and see how it goes, good to hear you know the ebay game. Don't bury yourself in fees trying to hold out for top dollar, I'm not sure on how much interest there is for this item, and something like this usually would sell easier around christmas as a gift for a collector's desk, than at other times of the year.
I looked at HCC rare coins in Toledo, their website is empty buy they have an ebay store, nothing like this is listed on their store.
I don't know what you paid for it but I'd think by looking at sold items on ebay of "coins in lucite cube", $20-$30 seems where they can actually sell. if they were real 90% silver versions, I'd say you could get more, but it isn't. Hopefully from Goodwill you got it for a couple bucks, maybe a $5. LOL.