Professional sales people will not talk down the competition because they know it hurts their own reputation rather than improving it. However, a pro may say in passing, "Have you heard about frames breaking on GM trucks that are being used for plowing?" A true thing purposefully thrown out to cast doubt on a competitive line-up.
And having a background in sales myself, I have no issue with bringing real facts about the competition into any discussion. With that said, just making statement such as "I wouldn't buy Korean" and "Hope you never plan on selling it" don't cut it for me. Statement that make unverifiable claims such as "I've heard they are made cheap" but then can't be backed up by facts and examples are lame, useless, and quite detractory toward my faith in the salesmans knowledge of the tractor market, and thus, his own business. Good salespeople, know their product and their competition, and can speak to both equally well. If you can't do that, then just keep your mouth shut about the competition and quote me your lowest price.