A: Most modern (post 1940) First Day Covers that are sold by subscription (e.g. Artmaster, Art Craft, Fleetwood) are produced in large quantities and have a minimal resale value.  Some dealers advertise that they buy these covers from 15-25 cents per cover if they are in good condition and are not addressed.  Earlier covers, covers of historic interest (e.g.  WWII covers, early century correspondence, etc.) are collectable and can be valuable depending on their condition and their origin.