I appreciate that viewpoint but I somewhat disagree.
In my view, a good deal is between two informed parties who, for whatever reason, value something differently; this difference in value is the fundamental basis for economic growth and prosperity ("The division of labour, from which so many advantages are derived, is the necessary consequence of the human propensity to truck, barter, and exchange one thing for another." - Adam Smith).
I think what some people are looking for when hunting on
eBay is not a good deal but rather a lucky windfall. In such cases it's not the seller's differing values but rather more simply their ignorance of what they have that provides the profit in the exchange, with the entirety of that profit going to the buyer. I personally don't feel all that comfortable exploiting someone's ignorance. A good deal means both sides are better off. Buying treasures for junk prices leaves the seller worse off, much worse off, and it's only the buyer's silence that keeps them from knowing it.
I'm not trying to argue that I'm right; I'm genuinely interested in other people's thoughts, and in particular their thoughts about where the line between a deal and a steal lies, if indeed any such line exists.