Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

How profitable is selling your customer base? 
There are business that exist by harvesting leads and selling them to other companies.  These leads can be access to resumes they sell to business looking for employees; they can be eyeballs that view their adds; they can be list of people that meet a specific credit profile. All are legitimate business and many are growing businesses. But in all these cases they are upfront with the things they are doing. They all have escape mechanisms for you to either stop them from selling your info to other customers, or to restrict the ability of those customers to contact you. There are also companies that are less honest with their collecting and selling of information. They are not honest about what they are collecting, and they have no care about how others use it. There are also cases where when a company buys another company, and  one main item in the transaction is the current and potential list of customers. Business with a legitimate product to sell, protect that customer list, that is the keys to the kingdom. They are the likely people who will buy the next version; they are also the ones that their competitors would love to target to convert them to another product.  In some businesses, the company that develops the platform will sell to developers of add ons access to the marketplace. They may charge a flat fee for access, or charge a percentage of sales, or both.  What you can do, and how you are allowed to do it, and what mechanisms are in place to protect people, are dependent on the country you operate in.