Sales doesn't need to be manipulatieve, it can be respectful. Here are some pointers.
Don't approach everyone
Personally, I don't like to be approached by sales people, let alone telemarketers. I like to have the initiative when it comes to making a purchase. I feel less respected when I'm approached before I have even decided I'd like to have some information, or have decided to make a purchase.
Sales people who do approach people are used to the response that the person "is just looking." How much sales persons like to convert those lookers to buyers, persuading people into buying, let alone manipulating them into that, is not really respectful.
Approaching people over the phone, without them even having a relationship to the company, is not particularly a sign of respect. Telemarketing companies in the US are now expecting to go out of business, because of the Do Not Call register. It shows that they never had a respectful position to begin with.
True for the customer
It goes without saying that when people ask information from sales people, that they expect to receive an answer that is beneficial to them, and not just to the company. Sales people getting bonuses the height of which depend on how much they sell, puts them into a position that it is profitable to them to lie to customers in order to get that sale. Customers shouldn't have to deal with such an attitude.