Chances are:
a) If you take up the PI offer, you might be able to bargain for a
higher trade in price since he is earning on the other end by
selling you the car.
b) You might be able to sell the car at a slightly higher price by
selling to a 3rd party but do remember: the PI agent may not offer
you a good trade-in offer as a result.
Have you tried calling other dealers to check out how much your car
is worth? If the difference is not great, might as well sell to the
PI agent who will do the paperwork on both ends for you, settle all
the financing and full settlement and chances are you will be able
to drive the old car till the very day your new car comes in. Saves
a lot of trouble in my opinion 
My 2 cents worth.
