Emailed to me from another humor list (Jokes & Trivia)
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Cards offering used textbooks for sale are posted on the college notice board at the beginning of each semester. One read: "Introduction to Psychology, $8, never used." The card was signed, "Must sell."
The next day a note had been added: "Good price. Are you sure it's never been used?" Signed, "Prospective buyer."
Below in a different hand was: "Positive!" Signed, "Professor who graded his exam."