The value, according to NADA, for an '89 560SEL with 100K miles is anywhere from $10K - $14K USD.
If that 560SEL actually sold for $3K, then I'm sure it was a salvage car. Or a parts car. But what the heck, it's the same thing.
*maybe the engine is on it's last leg, due to neglected servicing?
*maybe the body is rotting out, but they just don't show you it in the pictures?
*maybe the interior, in certain places, looks like a dog chewed off parts of the uphoulstery?
* what about the rear end? Hydropneumatic suspension shocks? Idle control sensor? All the windows work? Passenger mirror works? Sunroof? AC? Blah, Blah, blah...
As you can see, there are many different factors to consider.
You may pay more locally, but at least you'll know what you're buying!