It depends from salesman to salesman though, you have those who just want to sell you something you could use and you have those who try to sell you whatever most benefit is to their own pockets.
I good salesman can sell you something you need and not just the most expensive...you also trust the company more if you feel they aren't emptying your pockets.
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How much you earn maybe, it's a first I hear of people thinking how much their electric bills are considered private information. How can someone offer you a better price and service if you aren't willing to give information they need to calculate it.
Not all door salesman are scammers. If you notice they are they fly out the door though. :P