A few things to consider are: Are there known repros, mounting hole size and location, style of grommets (if used at all), dimensions, single or double sided, font, color, how/where the sign would have been used.
What are your concerns about the signs authenticity?
What someone says it's worth does not determine if it's a good deal or not. If it would put a smile on your face to own it at that price then yes, it's a good buy.
Wanted: Gas pump globes:Sinclair & affiliates, IL companies. Ripple bodies. Anything Sinclair, Stoll, Pierce, 4 Bros.http://www.lastgas15.com/