LOS ANGELES (REUTERS).- A 1942 piece of Superman comic book cover art goes up for auction for the first time on Wednesday in a thriving collectibles market where old superheroes can change hands for more than $1 million. The cover art for the Superman 14 issue, also known as the Superman Patriotic Shield cover, is described by auction house ComicConnect.com as “by far the single most valuable piece of original comic book artwork still in existence.” Illustrated by DC artist Fred Ray, the sketch shows Superman with an eagle on one arm in front of a stars and stripes shield and is regarded as defining the action hero’s status as a 20th century American icon. Steven Fishler, chief executive of ComicConnect.com, told Reuters the piece should conservatively fetch upward of $500,000 but that its eventual price was “pure conjecture” because it is so rare. Earlier this year, two comic books featuring the first appearances of Superman in 1938 and Batman in 1939 set auction sales records