Two weeks prior to the updated date when reveals were meant to begin for Commander Legends, a listing on eBay leaked multiple cards from the set. The cards were printed in German, and screenshots from listing have been published on Imgur.
It is TCGplayer policy not to list cards on the marketplace until after they have been revealed through official sources.
These leaks, which seem to showcase new and reprinted cards alike, were shown in an eBay listing which appears to have been sold. According to the seller:
This bid is for 13 booster packs. Each pack has 15 cards !!! Have 26 available and lot of 50 loose cards. All in mint condition of the same collection Commander Legends get them before the official release... DM if you want to talk about the loose cards.
Leaks in Magic: The Gathering are not a new occurrence. More prolific leaks have happened as early as New Phyrexia, perhaps even earlier. New Phyrexia was leaked in 2011 when a pair of professional players, Guillaume Matignon and Guillaume Wafo-Tapo, got ahold of the New Phyrexia "God Book," which contained the full set's contents. Martial Moreau and David Gauthier were also confirmed as involved. Since then, Wizards of the Coast has tightened their grip on such confidential information.
They do seem to occur more frequently than those days, however, but this may simply be because of the increasing amount of new cards Wizards of the Coast releases each year. Still, other leaks more prolific than this one have happened in 2018 with a full set leak of Dominaria. Recently, Crawling Barrens was leaked prior to the spoiler season of Zendikar Rising, and soon into that season, my own reveal of Obuun, Mul Daya Ancestor was leaked by Gamestop, of all places.
But I am not alone in losing out on leaks. The Professor's reveal of Oubliette, a major reprint from Double Masters, was leaked. While this led to some additional material to speak on in the official reveal on YouTube, it was also seemingly very disheartening to him on a more immediate level. Clearly, these leaks have their effect on content creation.
While it is possible that these leaks could have come from a product delivered prematurely to someone, it's also possible that these leaks from yesterday were conducted from the manufacturer's side of things.
All in all, this is a big story that may cause Wizards of the Coast to tighten the reins further on their unreleased product. As for what can be done by players, while we won't tell you not to look at leaks, I'd like to recommend perusing official reveal articles and videos by content creators when they happen.