Do YOU think it is ethical to use the relationship U have built over the years with ur customers in a giving company working as sales, in ur new JOB?

it depends on the market law, does it allow you to work with competitors after leaving your company or not, but Generally those customers are your own wealth and you spent a lot of effort to establish your network, and if you have strong relationship with them,it is fine for offering them new service or products.

Every product and offering have unique features. With new company a sales manager has new set of tools to make a case. If sales manager has been unethical in telling features of competition - then it will become harder to sell from new company. Sales has to be ethical and has to be competitive.