"In your retail out let: Optical shop; a stranger customer explained all the technical details of a particular glass". (Please follow the additions)

Big question and answer also need to be elaborated:-

First you should go to the 'stranger' customer introduce yourself as owner (it will create psychological impact with the customer) of the shop and ask for his requirement. Then try to use your technical details/marketing skills genuinely and sell the product. Your image will also increase with the other regular customer (who also will be watching this conversation). The third person's (If he is regular customer) interference will definitely an added benefit.. At the same time you should take the control and not the third person to talk more as again this will create doubt with the stranger customer. But,ensure the stranger customer should not feel you are too much pacifying him because if you cross the limit it will look like you are pleading with him to buy. Still if he is not convinced don't bother and just concentrate other customer(s). We can' t expect all the 'window shoppers' or 'stranger customer' can be your prospects or buyers. 

My important evaluation will be 'to make my staff more equipped with technical details of the product'.

Fist I must be convinced with my product. Should also try using the product . In a technical helpdesk where up selling is a part i should be using the moble phone or laptop or touchscreen. When a third person has a query should be able to answer him  so that the cusomer is not disturbed.

Eg if there is a objection to the touch screen app that the finger would pain after sometime I would offer the customer a wireless eternal mouse which would make the user comfortable when using the computer for a long time.

If I was the seller, I will neither accept or reject the interference (as you put it) of the stranger customer. I will follow the "Let buyer beware" principle. I will neither reinforce or reject the opinions of the stranger, unless I was sure that what he says is wrong.

As a principle we should encourage people expressing opinions, 

the interference here will has a sence only when the technical details are mis-explained.

Some customers have more inside on a product than some sales people. It is sad for me to see when it happens .. tells me the sales person does not know what he is selling or is just pushing his product even if he knows it is in lesser value. In this case I say thank you and leave to go shopping in an other shop where the sales force is better educated 

I echo with Mr. Mukesh Varadhan

This is situation based question, and there may be tens of variations in answers. There is no right and wrong answer.

There are other factors before we decide to have interference as owner.

What was the influence on other customers?

What was the intention of that customer who explained everything, expecting discount ..?

If sales team could not convince him, it is not a failure at their side, since the person was not open to other opinion.

If the owner's involvement is really necessary I would suggest to offer the customer a seat in his office, but this will happen only when situation seems to be going out of control.

If the customer is leaving without any substantial disruption, I will let him go and discuss the situation with sales team for ' lesson learned' for future.

agreed with all

Mr. Vinod correct answer

I believe as a owner, i will first try to evaluate if the 'Mr. Stranger' is giving the right opinion to the customer & if he is giving the right information i will add on by giving more details, user friendliness, special editions (Like Scratch free, warranty available IF ANY).

I will also evaluate how my sales person has demonstrated to the customer "As only because the customer didn't buy it does not make sense that Salesmen was not good at convincing".

I will also try to understand if the Customer's intention of visit was 'Just for window shopping or for actual purchase with eye tested details' & most important "Ability to Pay" as just "Willingness" will not stimulate the decision of buying it & neither i can sell at loss of margin.

Thus, taking everything into account will decide how i would convince if he is really come to buy & has ability to pay by helping him provide more options along with it's specifications & close the sale.

:-)

This portrays the deficiencies of many retails business. Mr. Third customer indirectly tried  to convey the message to the Shop Manager.

Perhaps,  the Stranger customer may be a good professional "Optometrist" seeking a suitable job in his field.

Thank you for all  your excellent  responses.