You may be going out to sell an unworthy, misery furthering product or service.
Do not think that you have made the fool of the customer.
You have made a fool of yourself.
And on the other hand, if you are able to sell a product which gives joy, and alleviates suffering, then you have given something of value not only to the customer but also to yourself.
By helping others, you help yourself.
By sinking others, you sink yourself.