I got my navigation SD card from eBay about two months ago when I though it was worth taking a chance for $45 when I was in the market for a Mazda 3. Ended up getting a CX-30, which uses the same part number (there is a new suffix now, but the original part number worked in my case). At that time there were a number of sellers selling the same part number for around the same price.
The packaging and card looked legit and the card has worked flawlessly. I updated the card to the December maps which was straightforward. As to ”what’s the catch“, I can only imagine how the sellers acquired merchandise. While the navigation is nice, I tend to use Waze or Apple Maps (really liking the new stop light/stop sigh references on the maps and directions...”turn right at the next light”). I really like how the Mazda navigation allows the speed limit signs to show up in the heads up display. Unfortunately the Mazda navigation data base (Here?) isn’t as complete or accurate as Apple Maps or Waze.