There are several resources that are publicly available where you can see what a car sold for at public auction. I subscribe to two reputable services called HI-Bid and Hammer Price.  These services aggregate public auction results from a global viewpoint.  I will find a comparable vehicle to your that has sold recently and then use the sale price to assist me in placing a fair market value for your classic.  It will be my judgment as to how your classis compares to the comparable.  I never use advertisements for classic cars as they are typically too high and they are often outdated.  Online classic car websites will leave ads up on their websites to give the impression that they have many cars available.  I might have to use or eBay Motors as resource but their sales conclusions are not verifiable.  In cases where the classic is extremely rare and no recent public auctions have taken place recently I will contact experts and ask them for guidance.  If that is the case I will document my research and sources.