In a fallen world Christians will tend to be naive and gullible. We are lovers of people and some of the characteristics of love is that we "think no evil,...beareth all things, believeth all things, hopeth all things, endureth all things. (1Corinthians 13 KJV)
We will assume that the person trying to sell us something is telling us the truth about what they are selling, we assume that they are not trying to cheat us, we assume that the person we are talking to is of high moral character. We will believe the best of that salesman, even when his intentions may be dishonorable. We trust that person, for that is what love does.
With that said many of us have learned the hard way. All people do not necessarily have good intentions and thus they may be out to cheat us. Thus we must take heed to the words of Jesus found in Matthew 10:16 Behold, I send you forth as sheep in the midst of wolves: be ye therefore wise as serpents, and harmless as doves.