That won't work. Trademarks are a lot harder to establish -- it's not just automatic from the moment you publish it as with copyright. You have to first start using it, then always mention it's a trademark when using it (with the TM or R symbols for example), then wait for it to catch on, then file with the government some paperwork. (Iirc, exact process is probably similar but different, but the point is it's a lot more involved)
Trademark is intended to protect the holder from being impersonated, not from losing revenue from selling content.... So it's a lot easier to redistribute copies of trademarked work as long as you make it clear you are not affiliated with the trademark holder, in a manner which a reasonable person would heed.
So for example, if a piece of art is trademarked by Disney, and it is well known by the public, and I print a copy and put it in front of my shop, a reasonable observer might this my shop is owned, operated, or endorsed by Fisney. So that's not OK.
If instead I sell copies of that art in my shop, and make it clear to everyone I sell it to that I am in no way affiliated with Disney and this is totally unauthorized by Disney, I'm probably fine.
Trademarks are also industry specific. That's why Apple Records and Apple Computer both exist -- as long as a reasonable person could not confuse them, it's OK.
In short, trademarks are very very different from copyrights. They protect different activities.
In fact I should not have used the phrase trademarked work. A work (like an image or movie or novel or software program) does not get trademarked. The character, slogan, logo, product name, company name, brand name, color scheme, etc used therein to identify the brand, is what is trademarked. Very different.
I will add more examples, this time to illustrate copyright, which works basically the opposite : Suppose mickey mouse were not trademarked. Then while it would be illegal to redistribute verbatim copies of a recent Mickey mouse picture authored by Disney, as well as any modified remixed versions based on that verbatim picture, it would be perfectly legal to draw totally new art involving the same character as long as it was completely new without referring to the copyrighted work, because coypright protects the right of Disney to make money off distributing that picture they made, and they did not make or contribute to making your mickey drawing, and while you are using a character they came up with, in the absence of trademark, copyright isn't intended to protect the public from being confused as to who they are dealing with as trademark is.
IANAL this is based on decades of amateur interest in IP law.
This is not 100% accurate from a trademark perspective, at least with respect to "famous" marks.
Generally speaking, you are correct - unless there is a likelihood of consumer confusion, you are free to use a trademark already used by a senior user.
But marks like Apple and Mickey Mouse, from a trademark, are sufficiently famous that they get special protection. There is a concept called trademark dilution that only applies to sufficiently famous marks. With respect to such marks, a junior user can be liable for use of the mark even if there is no likelihood of confusion.
(BTW: By "senior" user, I means a user that gained trademark rights first and a "junior" user is one that started using the mark in commerce later.)
From [1] "Not every character qualifies for trademark protection, however. For a character to be trademarked, the character cannot be too similar to other existing trademarked characters and must be used to brand products or services. Once a character meets these requirements, the owner can file for trademark protection."
So I don't know if you could apply trademark to e.g., every card in magic, but maybe only to the key characters?
Trademark is intended to protect the holder from being impersonated, not from losing revenue from selling content.... So it's a lot easier to redistribute copies of trademarked work as long as you make it clear you are not affiliated with the trademark holder, in a manner which a reasonable person would heed.
So for example, if a piece of art is trademarked by Disney, and it is well known by the public, and I print a copy and put it in front of my shop, a reasonable observer might this my shop is owned, operated, or endorsed by Fisney. So that's not OK.
If instead I sell copies of that art in my shop, and make it clear to everyone I sell it to that I am in no way affiliated with Disney and this is totally unauthorized by Disney, I'm probably fine.
Trademarks are also industry specific. That's why Apple Records and Apple Computer both exist -- as long as a reasonable person could not confuse them, it's OK.
In short, trademarks are very very different from copyrights. They protect different activities.
In fact I should not have used the phrase trademarked work. A work (like an image or movie or novel or software program) does not get trademarked. The character, slogan, logo, product name, company name, brand name, color scheme, etc used therein to identify the brand, is what is trademarked. Very different.
I will add more examples, this time to illustrate copyright, which works basically the opposite : Suppose mickey mouse were not trademarked. Then while it would be illegal to redistribute verbatim copies of a recent Mickey mouse picture authored by Disney, as well as any modified remixed versions based on that verbatim picture, it would be perfectly legal to draw totally new art involving the same character as long as it was completely new without referring to the copyrighted work, because coypright protects the right of Disney to make money off distributing that picture they made, and they did not make or contribute to making your mickey drawing, and while you are using a character they came up with, in the absence of trademark, copyright isn't intended to protect the public from being confused as to who they are dealing with as trademark is.
IANAL this is based on decades of amateur interest in IP law.