[rovr138]

Competitors, people actually wanting you to buy said product, the business selling since they get a cut, companies that process the transaction since they also make money. (no idea on regulations)

Why didn't you buy said product outright and instead put it on a wishlist? Lack of money? Price too high? Dubious quality?

[Pokepokalypse]

There's also the factor:

Advertisers core-competency is "selling"; often used to sell their own services. Sometimes using deception and fraud. So in addition to all of the other explanations on why we get inappropriate ads: some vendors may be paying advertisers to send out ineffective and inappropriate ads, because that advertiser conned them into buying that service by lying about how effective the ads or methods actually are.

I think this probably explains about 90% of all advertisements.