I tend to agree with this sentiment, though I have mixed feelings. Gearslutz has a policy that people are not allowed to complain about the price. Of course, they have moderators to enforce that policy, as well. Perhaps we should institute something similar here.
I do get upset when someone posts an outrageous price for something, though. For example, there are a couple of e-bay sellers who regularly sell new Audio Technica AT4050's for $399.99. Most online retailers sell it for $599. If someone posts a used AT4050 and wants $475 for it, that's their perogative, but I would feel bad seeing someone pay that much when they could get the same mic new for less money. However, I think this can be more tactfully handled by sending a pm, rather than publicly blasting the person for overcharging.
Personally, either here or on e-bay, I won't pay more than 75-80% of the price of the new item (shipping included) for used gear (of course, I would prefer to pay less). I think this is a reasonable discount for the risk of buying gear used and without a warranty. That's just my personal policy, though. As always, buyer beware. It is your responsibility to check around for the competitiveness of pricing and the reputation of the seller.