If you want to be viewed as an amateur who happens to sell products, you can act anyway you like. If you want to be viewed as a professional, the path is much straighter and narrower. E.g., your “we agree to disagree” is tactful and how a professional responds to an insinuation. There’s nothing wrong with just ignoring someone, if they’re being rude or aggressive or the like.
Yes, it sucks. Just keep in mind you’re the one who’s going to get paid. 
Okay, lecture over. No hard feelings, and sorry if I came on too strong.