OK, the dealer advertizes that they will give you what your used car invoiced for when it was new regardless of condition. This is obviously something other than what it appears, but what?

They may not come down on the price of what you are trading the old car for, or they may be trying to sell a gas guzzler, which would otherwise sit on the lot for a very long time. Trust car dealers as far as you can carry their product.

Negotiate the price BEFORE discussing the trade in- always!

Do your homework- check the price of comparable vehicles within 100 miles of your location through the newspaper, Kelly Blue Book and online resources like AutoTrader.com...Check the private sale value as well as the retail value and shoot for somewhere in the middle.
Besides the listed price,some things that you can usually get knocked down are "Dealer Prep", "Destination Charge"and other little additional fees that make no sense- There's no reason you should pay the dealer for having the car on the lot and ready to sell unless you special order it- If the car has been test driven a lot, there may be in excess of 200 miles on it, and you can use that to wrangle the first scheduled maintenence for free.