how can marketers gather essential information while repsecting the privacy rights of consumers?

Answer 1

This is easy. There is NO SUCH THING as essential information for marketers. Nearly any information is useful to them, but NONE is essential.

Answer 2

A lot of the information is gathered when you check out at the store. Like those Safeway Club Cards, they track your purchases and they got basic non invasive info when you filled out the registration form. Marketers like these gather an age group, income group and area, combine it with your purchases and bingo they have a demographic copy of you. All without asking your birthdate or occupation.