All this guff about the sale price being "private" or "none of your business" is just daft. This isn't some old fashioned gentleman's club nonsense, it's the Internet. Transparency helps create a proper marketplace where pricing is fair, and not overinflated. I find eBay's sold listings extremely helpful for determining what something is valued at, and you can then adjust your price up or down depending on the condition or specifics of your unit. While eBay's prices are often a bit higher than you'd expect, you can round down as necessary for forum sales as you're not paying fees and whatnot.
If you're advertising an item for £500 where identical ones might only have sold for £350 on numerous other occasions, then your item is overpriced. If your item then sells for £150 then there must have been a reason that you accepted £200 below market value. I see no point in hiding this. If you're selling something in a hurry because you need the money and will accept lower offers, who cares? If someone then either buys this cheap item, or manages to buy something from me at roughly market rate and then manages to sell it for a profit a week later, congratulations to them I guess. The more information there is, the better a seller can set their price based on averages rather than potentially ripping someone off.
Historic pricing information becomes even more useful in the case that you're selling a less "mainstream" piece of equipment as is quite common here.
Even more annoying is when someone sells something but doesn't know the value, so they invite offers. You're the seller, you need to determine a price and set one. Hence the above information becomes useful.