I'll take umbrage with the upsidedown buyout and zero interest things as
being "cons" or "scams".

It's just numbers.  There's no con in the numbers.  People do want to
take their upside deal and get rid of it, and that's a legitimate
service/product to offer.  It's the owner's problem that he's paying way
too much, not the car seller's problem.  What do you expect the car
seller to do--tell him no, that's not a good deal for you financially,
so I won't do it for you?  That's not the car seller's business.  His
business, his job, is to bring car buyers together with a car--period.  
His job isn't financial counseling, or passing judgment on the stupidity
of the people who insist on turning over upside down car after upside
down car.

Same with zero interest.  Sure, it's zero interest--as long as you pay
invoice.  Well, that's just a way to hit some peoples' comfort level.  
Some people respond to zero interest and never actually look at the
numbers.  Is that the problem of the guy selling the car?  Sure, such
people are stupid--but see above.

That people go out and buy cars and never actually crunch the numbers is
NOT the business of the car seller.  People in this country are free to
be as utterly stupid as they want to be with their money.