Once inexpensive and readily obtainable, authentic World War II items now sell very quickly - and at a premium. This is due mainly to superior craftmanship as well as the increasing rarity of genuine items. Decorations have been issued in the current plastic cases since the early- to mid-Vietnam Era (and the quality of materials and craftsmanship has steadily declined).
I dealt in military collectibles for years and I'm about as close to an "expert" (if there is, in fact, such a thing) as you'll find today. No more. It's incredibly easy to fill a garage, attic, basement, storefront, etc. with junk - any kind of junk - hoping that "somone" will buy it "someday".
Now and then I get a line on hard-to-find items that have a potentially fast "turn-over"; when this happens, it's "first come, first served". I don't save numbers and addresses of those who "might" buy something "someday" so the best I can do is suggest you check with me from time to time.
Re your Grandfather's three awards of the Bronze Star. The decoration should have two small bronze Oak Leaf Cluster devices affixed to the ribbon. These represent the 2d and 3d award. There may also be a bronze letter "V" device. This indicates at least one or more awards was for valor in combat (or in direct support of a combat operation; the Bronze Star may be awarded for combat or non-combat action).