After traveling through time back to April 18, 1938 to pick up several copies of the first issue of Action Comics, Doc Brown sold one at an unknown point in time to finance the hover conversion of his DeLorean, and to purchase a Mr. Fusion unit, which he installed in place of DeLorean time machine's nuclear reactor.
He then traveled to some point in the 21st century to Southby's Auction House. The dealer there purchased one of the issues for 2.5 million dollars, and intended to sell it himself for 3 million dollars. Once Doc was gone, he was discussing the purchase on the video telephone, through his video glasses. He insulted Doc for selling it for such a low price, not knowing that he had several more issues that he intended to sell later.