Some customers have better knowledge about the company products more than the employees whom meet them what do you advice them to do in such cases?

Don't try to be smart by trying to prove that you know more about your products. 

Try to get the customer involved in the product performance and development by asking for his feedback.

If you want to make an offer, just concentrate on the financial benefits of your offer not on the benefits of your product.

The staff member should stay humble and ask further questions from the customers of their wealth of knowledge of the products. so that they can learn a different sales pitch to sell to the next customer and this time round they will score the sale. as they are talking from a consumer point of view

True words and managers should be tested first before the appointed staff or trained in an excellent manner I.

they have. To use customer empowrement technique if the it is positive feed back regarding the product

they have to raise their objections and solve it if the feedback is negative

and in both cases they have to improve their product knowledge to meet the requirements of such customers