See, being community-spirited (and big-mouthed to boot), if I see somebody selling a 'new' CD for 4 pounds more than it can be bought new on the web including postage, I tend to point this out. I don't like seeing people getting ripped off, and besides, the guy selling it will presumably stand more chance of selling it if he has an idea of market prices. (Or maybe I'm deluding myself about the latter reason; even a quick glance at eBay makes it abundantly clear that it's possible to sell things for several times their market price if you can attract just two clueless bidders.)
I suppose getting told -- rudely and illiterately of course, but what do you expect from usenet? -- to mind my own business was inevitable. But this is the thing: I genuinely think it's everybody's business to be aware of, and increase the community's awareness of, the range of prices being charged on the wider market. Admittedly, sometimes you may have valid personal reasons not to buy a CD at the lowest price offered (perhaps you haven't yet stopped boycotting Amazon, or perhaps you want to buy direct from the band or artiste's website to show support for them, or perhaps you prefer to support your local indie record shop rather than buying from -- for example -- online indie store OpalMusic, not to mention the big mainstream stores like Play and CDWow. But generally there's no good reason to pay extra for the privilege of subsidising a private trader in a local small-ads group who already makes a tidy profit from reselling duty-free cigarettes on the newsgroup.
Now, obviously (caveat emptor!) the buyer has some responsibility to make themself aware of whether a quoted price is fair or not. If, that is, they care -- I assume some people are rich enough that they honestly don't care if they're paying several times over the odds for something. If they really don't care, though, they shouldn't really mind seeing the cheaper alternatives pointed out, any more than they mind, say, being aware that planes have an "economy class" cabin as well as their own luxurious Platinum Flyer class. But in this context, objections to information being freely provided can only really come from those who want to profit from other people's ignorance; and that's something that I and my big mouth are prepared to shout about.