I agree with many of the already existing answers, but I would like to add a little more...
In Star Wars' case The Death Star(s) can be seen as a an iconic visual prop (model, set piece), a character (because it has a distinct look/feel that helps shape both Star Wars aesthetically and because big events happen to main characters there, making it more than just a setting), and a possible (by George Lucas' standards) MacGuffin, as it is also a plot device in the form of a mass super weapon that is directly tied to the plot of both A New Hope and Return of the Jedi.
In addition, because the Death Star doesn't just appear once, but twice, it also then becomes an over arching theme through much of Star Wars, but by expanding on the idea of important space stations (See High Republic's Starlight Beacon and Amaxine Station) or other kinds of similar super weapons (see: Starkiller Base). These are some of things that make Star Wars, Star Wars.
The Millennium Falcon is similar, but thematically tends to be in association with the identity of certain characters (Han, Lando, Chewbacca, L3-37, Rey, etc) who come to own or helm it: playing to broader theme of 'space pirates' and the like.
It serves as an extension to the identity of the characters who fly/man the ship, but also for most of the protagonists, it's a place where those characters often grow together. In the case of Rey, we also can view the Millennium Falcon as a baton being passed, making the ship apart of Star Wars ongoing theme of family and legacy.
It may be more debatable to consider it a MacGuffin however, as although the smuggling chamber, an ability to get places, a capacity to fight back might all be seen as things integral to the plot, it is more defensive against the bigger Macguffin or existential threat. It may be integral to the plot, but it is not necessarily driving the plot.
Now to your update: when you selling/buying merchandise for any well-known film or franchise then you are selling/buying: toys, games, books, comics, clothing, home decor, kitchenware, and/or other kinds of memorabilia (models, replicas, posters, autographed items, film stills/animation cells, etc)