Who has told a buyer that what you have is not what they need?

I know this thread is more about car-related deals. However, with my business, I am always careful to make sure that I tell the potential customer if what they are asking for is really what they need. I sell custom automated machinery that can range from $10,000 up to Multi millions. It does me no good in the long run to sell the client something they do not need. In the long run, treating people (and other businesses) with integrity will attract more customers than a quick hit & run payday. This also helps me sleep at night!