Credulous

(남의 말등을)잘 믿는, 속기쉬운, 경솔히 잘 믿는데서 오는

credulous people who believe what the advertisements say
광고가 알리는 것을 믿고 잘 속아 넘어가는 사람들

Vic is so credulous that he actually believed me when I said that I had invented an automatic composition writing machine