The dealer will typically rape you on trade in value. They will most likely give you fair market trade in value which is substantially less than private party value. You are much better off selling it private party regardless of tax benefits.

Go to Kelly Blue Book to compare values, although, the dealers typically use NADA book values instead. KBB values tend to be "inflated".

I use to be a salesman at several dealers in town so I have pretty good experience.