Hello Azarashi,
"Japanese" by itself is grammatically correct. It works fine as a stand-alone demonym (the same for Italian, American, etc...). There are irregular demonyms where you would want to add something on the end - for example "English" doesn't work, but "Englishman" works. Japanese, however, is fine on its own.
Of course it wouldn't be incorrect to say "Japanese person", or "Japanese buyer" as you suggest. Some people might find it more politically correct to add such a word on to the end to soften the phrase (in effect, to humanize the person who is the doing the action, rather than just referring to the person as a nationality).
But for Hirashin's purposes, and really I think for just about any situation, both (a) and (c) would be completely fine, correct, understandable, and common.