Well the 1939 is old, but, it was a high mintage coin for that time period. 120 million. Even if the book value says it's worth a dime in that condition, there's no market for it. However, some 1939 nickels carry the reverse of 1940 and that's worth a little bit. Some also have the reverse of 1938. Also, the 1939 can have a doubled die on the reverse, that would be worth something. Odds are your nickel is worth a nickel, but it's good to know those other things, and what they look like and how to identify them. If you use the search box here, you will find photos, articles, descriptions of the things that I mentioned.
