...but when you say "withheld from the seller," you're implicitly assuming that the (also assumed) intended resale to VW is something that should be disclosed to the owner/seller. Why do you assume this?
You are assuming, without evidence, that Hyundai of Cookeville is aware of the details of the 3-liter buyback. You are further assuming, also without evidence, that this mailing was targeted to TDI owners. (And I was joining in those assumptions in my first post on this thread, before I'd thought much about it.) And finally, you're assuming, still without evidence, that their plan is to buy the TDIs to sell them back to VW. And on the basis of those assumptions, none of which is supported by a shred of evidence, you're saying they're wrong for doing what every single dealer in the country does, every single day: offering to buy a car for less than they believe they can sell it for.
So, despite your deflection, I think my question is well stated: at what point, or under what circumstances, does buying something with the intent to resell it for a profit become morally objectionable? What, in your opinion, is the ethical rule? Or are you unable or unwilling to articulate one beyond "I know it when I see it"?
My general rule is that transactions that are freely entered into between a willing buyer and a willing seller are presumptively ethical. However, there are factors that swing the needle a bit. Active misrepresentation of material facts swings the needle (hard) toward "unethical". Active concealment of material facts does the same. Withholding information that a party has a duty to disclose does the same. There are probably others, but these are the big ones. I don't see any of these in play here.