We should also look at the thought of asking the seller to quote the price at which the product was bought, it will provide an equal base/right for buyers as well since the seller is free to quote a price he/she deems fit.
Though this is a valid point (and probably the second most topic of posts in many For Sale thread), it would be difficult to enforce in many cases. I don't remember the price I bought many of the equipment that I bought. If I have to sell any of them then I have to make up a price in the absence of proof of purchase. That would defeat the purpose completely. So, I am not sure if it is good idea.
The problem may be with the expectation that buyers have a right to know the price at which the For Sale item is bought. I don't have that expectation. And I don't agree with that expectation. But if the majority feels that way, the mods can enforce it though IMHO such a requirement is not practical.
Also, many have said that if the seller is making profit it should be posted in the For Sale by Dealer forum. I thought that forum is for users who registered as Dealers (those that have a business of selling HiFi items) irrespective of profit or loss the pieces are sold at. May be the mods can clarify that.