I would never buy a car from ebay, unless I knew the seller. You never know what you could be getting into.
It pays to always see a car in person and drive it for yourself. Pictures are nice, but they're certainly not enough to showcase the car's possible 'hidden' potential.
An '89 560SEL that SOLD for $3K? Hmmm..., there must have been something wrong with that car, eh? People tend to be extremely conservative when spending money on ebay.
As far as values go. NADA is the book of choice for me. They have three prices based on condition of vehicle.
1987 300SDL (324000)
1986 Porsche 951 (944 Turbo) (166000)
1978 Porsche 924 (99000)
1996 Nissan Pathfinder R50 (201000)