Food Grade Sealant for Oyster Shells?

I'm not sure about using shells for foods, but I know we always boil our shells before using for any crafts.
I would suggest you go to a hardware store like Ace and ask for help.

I've always been able to get answers from my local stores.
You might try calling a restaurant supply store and ask if they have anything you could use.
There is also a government food site that has all types of booklets on how to do things like this. Not sure about shells but may be worth a try.
Sorry, you will have to do the research yourself, as not cleaning these properly could be a serious health hazard.