Step 1: Build a website that sells things. Step 2: Sell the data about those transactions to anyone and everyone willing to buy it.
After that, who cares what is done with it? Best not think about those things lest it bother your conscious.
Your question probably is more along the lines of why would someone want to buy that data? You add that info to the buyer's enormous trove of data profile. This person bought an Apple device = they have lots of disposable income (send them ads on other superfluous crap). This person bought a baby crib = send this person lots of adds about diapers, formula, toys, etc.
