With regard to the price for your dulcimer I think it is too high, but then you might find someone who is willing to pay that much. As was mentioned, there is nothing special about Lynn's signature in an instrument. As he told, whoever was tasked with attaching the dulcimer backs on a particular day signed the labels and glued them in before attaching the back. The Fun With The Dulcimer book isn't of much monetary value. Jean Ritchie's, The Dulcimer Book might bring $25 -@30 if sold separately (on a good day). Force's In Search of the Wild Dulcimer is available for free download online. I don't think the case adds much to the price. All told, the $400 - $500 price range seems reasonable but you will have to find someone who likes using wood tuning pegs or is willing to spend an additional $100 - $200 to upgrade to geared pegs. In checking the actual sale prices of used McSpadden dulcimers the vast majority of them are going in the range between $200 and $350.
Ken
"The dulcimer sings a sweet song."