There are a few threads about this topic already, but long answer short, the price is an agreement strictly between the seller and the buyer, at the moment of the sale. After the transaction is completed that price means absolutely nothing for the rest of the mortals, and most of the times the final price is not even close to the one listed, as the seller and the buyer agree on different terms, and people do it for privacy purposes...
Also keep in mind that the price of a used item, is a function of a lot of aspects of the market, and many of them could change from one day to the next...Availability, physical condition, desire of the buyer to have it, confidence on the seller, etc...
I paid for my CD3K while they were available around $350.00, right now the price for any used in good condition is about two times as much, so if I would going to take a look at any sale to form an opinion, I could not be more wrong...