I think there might be a culture clash between you and the grandparent.
As I understand it, no one is getting taken advantage of by the grandparent. eBay's a marketplace and the seller put their items up for bid. Fair dealing does not require a buyer to tell a seller what their item is worth to someone else. When a buyer gets a bargain, we celebrate.
Fair dealing does require sellers to be truthful about the quality of their goods though. Lying about a lemon and pawning it off onto someone then profiting off that lie is a bad deal.
I'm speaking as an American, and that's my understanding of how we trade. Your word is your bond, handshake deals are as good as a written contract - that sort of honor code is pervasive.
You should mind your own business. I mean that to be taken politely and not as a slam. There's just no better way to say it, so I said it. There is not enough information in your sentence for me to guess at what your thought process was, so I'm going to assume you were just expressing empathy and trying to be nice.
I'd like to hear a moral justification for how you came to judge the grandparent.