When you trade a vehicle, the tax is only on the residual amount owed. So if you trade in a $3000 car on a $25,000 car, you pay tax on the $22,000.
I think that with a private, straight trade, they will look at book values to establish values, and then tax based on that.
Check with the Motor Vehicle branch to see if you can trade "as is" vehicles...
BMW makes GM look better every day