Never ass-u-me that a used bike at a bike shop has been fully serviced. I've seen a ton of as is bikes, obviously neglected, obviously not maintained, for sale at bike shops. Buyer beware.
$500 for a vintage bike is the higher end for sure. Has to be pretty special to command that price.
You mention you are not knowledgeable about vintage bike values. In any financial transaction, the one with the most knowledge WINS. Doesn't matter what you are buying.