There are several resources that are publicly available where you can see what a similar vehicle sold for at public auction. I subscribe to reputable services that collect public auction results.  I will find a comparable vehicle that has sold recently and then use the sale price to assist me in placing a value for your classic.  It will be my judgment as to how your classic compares to the comparable vehicle.  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 www.Bringatrailer.com 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.