What if the age of the item is unknown? Granted, many model numbers change every 2-3 years so you can somewhat figure it out, but many do not. I have bought used gear where the seller did not know how old it was, how am I to know how old it is then when I sell it?
As for your #2 concern, when posting an ad, the Audiogon system ask that you post a MSRP price (if you know it). They do not ask that you post the price that you actually paid for the item. It is up to the buyer to understand the market.