Is it actually legal to sell customer data in India by tech companies?

I have been wondering how these startups make money. Pretty sure they sell data.

I just want to know if there is a legal framework which allows them to do so? I am asking specifically from Indian market's point of view.

Can a company's source of revenue be from selling customer data (legally)?

What if customer of an app XYZ voluntarily agrees for their data to be sold in return for some rewards or maybe even without rewards. Please share your views and facts about the same.

If someone has any idea, can they shed some light upon a rough price range in which the data is sold. Like I want an idea what's the quantum of cost of that data. Mis it in 100s per user or thousands or what exactly. Just a rough idea.