Hey @FantasyHaven,
You should, in theory, only have to select your files (.uproject file, content folder, and config folder) and zip them into a folder.
A great way to test and see what the marketplace might be seeing is to zip your files like described above, move them to a different folder, unpack them, and try to open your .uproject and see what happens.
Definitely let us know what the result is, we can figure this out!