The first thing a salesperson should do in dealing with a customer's complaint is:The first thing a salesperson should do in dealing with a customer's complaint is:

The first thing a salesperson should do when dealing with a customer's complain  is to be very attentive and don't react quickly. patience  is the keyword.

Explanation:

At first when a customer begins to complain it is always advised to have a listening ear to a  customer to understand their problems they might have face over something. That is, by putting yourself in their position, the salesperson should try and pay attention and make a big difference for the customer.

Most sales personnel would want to react at first, but it is important to listen and see the customer's point of view before reacting, and if one is care enough, he or she can be of assistance to the customer, by looking for better alternatives in solving the customers problems.

We have to take into consideration first, that customers are human beings and we need to be of service to them, in whatever situation they face, the sales personnel is meant to solve the problem. a balance needs to be created for a better environment for both the customer and salesperson.

The salesperson must always be on the same team with a customer for example if  a product bought by a customer wants to be returned back, instead of reacting in a bad light like saying, "the product cannot be taken back", it might go in better tone as "it looks the purchase date of return of product has expired, but let me see what what can be done by the management".

i would say a penny hope it you