curious what it's signed on if it's a greeting card and there covering up an inscription or something I would say there is a shot at it being good but if it's just a piece of paper then it likely isn't as he would not sign autographs so it's more unlikely to not only be signed but have with best wishes.
+ 1. Also there could be a number of not so innocent explanations as to why it has stuck down and framed? Is it from a dealer on ebay or a Salinger fan?