Well nobody who trades in a car they bought new one year later cares about depreciation anyway, except to complain about it, and use his "past/recent performance" to project poor "future results", LOL
And there is this: A car salesman has no vested interest in paying you top dollar for your trade, or in reducing the cost of the new car any more than is necessary for you to sign the deal.
They are therefore completely unreliable sources of information as it pertains to how well your trade will sell, how many of your same used car (or better) are sitting at every other dealers lot in the nation unsold and unwanted, etc.
They have a dog in the fight, they are not neutral and objective sources of beneficial information. Edmunds is a more reliable and objective source, or Consumer Reports.