i-adjective
gullible; naive; easily deceived
Describes a person who is easily tricked or believes things too readily. Often used in warnings or self-deprecating remarks.
See also: 騙す
彼はだまされやすいから、気をつけてあげて。
He's gullible, so please look out for him.
そんなうまい話に乗るなんて、私もだまされやすいなあ。
Falling for such a sweet deal—I'm so naive.
騙す is the transitive verb 'to deceive', while だまされやすい is an adjective describing a person who is easily deceived.
素直 means 'obedient' or 'honest' in a positive sense, whereas だまされやすい implies a vulnerability to being tricked.
Compound of the passive stem だまされ (from 騙す 'to deceive') and the adjective やすい 'easy', literally 'easy to be deceived'.