fleeceable, naif, naive, unwary, green, gullable, cullible, gullish, weak-minded, soft-minded, more...

skeptical, pessimistic, cynical, doubtful, wary

naive, credulous, unsuspecting, trusting, easily fooled, susceptible, uncritical, unskeptical, ingenuous, more...

easily manipulated,   too trusting,   easily persuaded,   easily convinced,   easy to convince,   overly trusting,   easily fooled,   easily led,   like a silly bird,   easily duped fellow