too trusting and easily fooled
gullible - naive
If somebody is described as being ‘gullible’, then they are thought to be too willing to believe things or too trusting of people. Even though it can imply innocence, which is a word with some positive connotations, it is always used in the negative because gullibility is associated with foolishness and a type of naivety that can be costly for yourself and others.
Gary was tricked into giving his life savings to a con artist and now he and his family are destitute. How could he be so gullible?
The street vendors selling fake antiques in that city rely on the gullibility of tourists to make money.
I gullibly took the client at her word rather than putting the deal down on paper. Now I’m out £50,000.