Selling your Customers...

Every large or medium scale services company has a loyal base of customers, and for god's sake, there are a billion services outside IT Services, so please do not assume customers to be mega firms here!

Imagine a locality or a town or a village being served by a firm. Now, that firm would automatically want to stretch its revenues and looks for all avenues to boost them! In that regard, can it collect the info of its customers and sell it to interested firms?

Suppose a firm A owns a cattle farm and provides daily milk to an entire village V. Now, can A ask every interested family in V to fill a form giving out contact and demographic details, compile that data and sell it to banks or other interested firms?

This is seemingly an easy task, but what if later families of V sue A? What are the legalities involved in this sort of data collection? Can such data whose intention of collection is not divulged at the time of collection be sold at free-will? I have not come across any law that curbs such acts, but I dont feel it ethical to do so. If you know of any law, please do tell me and if u just feel something about it, let me know!

The most important point that hits me is, in case it is legal to do all this, should A inform residents of V of its intentions beforehand? Also, in this age of protecting consumer privacy, are there any legislations anticipated in this regard?