Consultative selling is achieved by asking contrarian questions i.e. those which take an opposing view and reject the majority opinion.
Making this a sales tip is a little dangerous, especially as a short one without a sufficient explanation. So, let me expound a little. Sales is all about helping customers either fill needs or correct pains they have. When you take the time to ask questions that expose an opinion or view that is clearly different from what the customer is probably thinking, you have an opportunity to get the customer to see a bigger need or pain. This approach often opens the door to a sale much larger than what was initially expected. It is most effective when you encounter a customer who is already predisposed to buy. By asking them a contrarian question, you have the opportunity to expose additional potential.
