The key word in your explanation of how the quoted sentence was used is "quoted." Anytime you quote someone or something used in another source, you should include a citation and for clarification, include an attribution. For example, you could have written it as follows: John Smith wrote in his article that "there are no sellers without buyers" (citation). Different documentation styles (MLA, APA, etc.) may handle the citation differently, too. If you quote, you cite. Simple. Even if it's a common quotable quote.