Action Comics is an American comic book series by Detective Comics, now known as DC Comics. Issue #1, released on April 18, 1938 (cover dated June 1938), introduced Superman, cost 10¢ and is now one of the most highly sought after comics in the collector's market.
On August 8, 2015, Doc Brown won first prize at the Hill Valley 1980s Car Expo with his DeLorean, which was 10% off a hover conversion at Goldie Wilson Hover Conversion Systems. To get the money he needed for this and the Mr. Fusion, he searched the internet for the best investment to make, then traveled back to April 18, 1938 to pick up several new copies of Action Comics issue #1.
He sold one in 2015 or beyond to get the money he needed, and then traveled to 2015 or beyond, and sold one at Southby's Auction House for $2.5,000,000 million dollars.