I think the point is this:
If you want to post a price check in here to gauge if something you want to buy is priced right, that is probably a good idea, before you go ahead with the purchase.
If you need to check the value of something before you sell it, that's a good idea, also.
After you purchase something, coming here to see if you got a good deal or not is maybe not wise. You will either confirm that you got a fair deal, find out you got an amazing bargain (in which case the seller might find this thread and be filled with regret), or find out you overpaid (in which case you will be the one filled with regret). Once you as a buyer and the seller come to an agreement on price, that IS the value of the item. Something is worth exactly what one person is willing to sell it for, and another person is willing to pay for it.