As both a marketer and a developer, I can say that buying an email dump is NEVER something that I would do, but not its for the ethics of it (we all know marketers have no ethics). I wouldn't buy a list to spam because THAT list isn't MY list.
The hit/conversion rate on a list of random emails, or even semi-qualified emails, that you don't have a relationship with is so incredibly low, that it isn't worth it to spam their inbox, risking my reputation (opens & clicks to spam ratio).
A one percent uptick in spam reports is enough to downgrade my sending IP enough that my real customers would start missing my emails.
Real marketers aren't going to send a automated marketing email to a list of people they don't have a relationship with.