From February 1999 (and the very outer limits of the Star Age): the mildly historic last issue of the MARVEL MONTHLY CATALOG.
It's significant because it marked then end of Marvel's misplaced ambitions to distribute its own products to the Direct Sales sector. The ambitious suits within the burgeoning comics-to-cards empire thought they could cut out the middle man by ditching comicdom's traditional wholesalers (who were at least more efficient, and less corrupt, than the news stand suppliers that publishers had wrestled with in previous decades) and offering product direct to the retailers... on Marvel's terms.
That legendary misstep sparked the short-lived Distributor Wars which saw Diamond and Capital fight to sign exclusive deals with the other publishers. Diamond did well. Capital less so. And the other, smaller, players were squeezed out entirely. Capital eventually conceded defeat and merged with Diamond, leaving just two (the other being Marvel) to slug it out.
Marvel's HEROES WORLD subsidery never nailed it and, as the losses mounted, Marvel also folded and... with no other option... were forced to go back to the only other player in town. Marvel's greed and ineptitude had shaken out the industry... and left it going cap-in-hand to the monolith it had helped to create.
This issue was the last to appear as a standalone magazine-cum-catalog. From the following month, Marvel's books were once again listed inside (or, more accurately, as a standalone insert) in the main Diamond PREVIEWS phonebook-a-like (albeit much diminished from the whoppers that appeared during the boom years).