As proof of this statement, a good many years ago, an antique Uncle Sam dressed as Santa Claus postcard brought , ( if I remember correctly ) over $ 6,000. The picture of the postcard was on the back cover of Mash's postcard reference book that year. And, the amount was due to a bidding war.
Although a very rare card indeed, it would take another auction with at least 2 high end postcard collectors at war for the card to generate that price again.
It is thanks to watching Charlie long ago, proclaiming to his numerous victims through the years, " you paid too much ", that I learned how to avoid the bidding war.