According to another poster the 88 300TE does not have the self levelling suspension; that was 89 or later. The wholesale value for trade in assumes the dealer will do the usual things like brakes, tires, detailing, battery, basic service so it is ready to run. Private party assumes that not as much of this stuff has been done AND that you have zero recourse, for the most part, with a private party.
I would want to know if the top end (at a minimum) has been rebuilt with that kind of mileage on a single cam 300E motor. The tranny could also be ripe for replacement between 200 and 300 (mine was done at 220 on my 300D) and that will set you back around 4k for a warrantied unit from Mercedes.