Though View-Masters today are mainly considered children's toys, when they were first produced in the 1930s, they were marketed to adults as a more modern (and fairly upscale) version of the stereoscopes, which had been popular since the mid-1800s.
Most View-Master reels, even old ones, are very affordable today — even the more "valuable" three-reel sets generally sell in the $10 to $50 range — but some of them are far pricier. Probably the most valuable mass-marketed View Master reels are the "mushroom" reels: A complete 33-reel album of "Mushrooms in their Natural Habitats" can sell for over $1,000 today.
If your father or grandfather fought in World War II, see if he has any reels in his attic — during the war, the U.S. military used View-Masters and reels to teach soldiers how to identify various models of aircraft and ammunition.