Subscribe to The Chicago Manual of Style Online
Q. Our marketing people want to know which is correct: The buyer(s) purchase a policy or the buyer(s) purchases a policy.
A. Neither is correct. The optional plural doesn’t work as the subject of a sentence; it works only as an object: “The title, along with the name(s) of the editor(s), appears on page 3.” One solution is to spell out your meaning and make the verb agree with the nearer subject: The buyer or buyers purchase a policy.