Wow, thats a nice one. Its got whats called the Olympia stock on it, mine is a UIT competiton stock.
Don't take it to a gunsmith, most don't have a clue about air rifles and may do more harm than good. Although it may look like a fire arm it has little in common with one.
As for value, make room in the safe for it. Its a collector piece, probably worth something north of $500, maybe quite more a bit if its near 100% and unfired.
If you want more info, ask the experts on the American Vintage Airgun Forum. They specialize in stuff like this, and there are several Walther collectors there that will fill you in.