In “Objective IELTS” by M. Black (Teacher’s book) the word sales is used without articles, e.g.: “In the first quarter, sales of the new “Flying Boots” were …” “… while sales of “Zed Runners”…” et c. (from a sample essay on trainers sales). Is it wrong?
Not all nouns require articles in English. Specifically, when one is referring to a class of things (in this example, sales) no article is required; the noun is being used as though it were a mass (uncountable; non-count) noun. To the extent that there's an implicit quantifier, it's context-dependent but usually one of "all", "most", or a specifier such as "the relevant".