$18K is too much money. Just pulling up Auto Trader listings shows numerous 2004 C320 4Matics with 60-70K miles listing anywhere from $13,900 to $16,900, and that's list price before you start negotiating. I think they're being fair on your car. Auto Trader shows several 2000 C280s with 80-95K miles listing anywhere from $6900 to $9K - there are a few more expensive ones, but again, that's list price and its before any overhead-related costs have been applied to the vehicle.

If you're concerned about the W203's owner history then walk away. W203's represent a bit of a low point in MB quality control. But if its just about the $400 I'd go back and try and split the difference with the guy. Don't get hung up on the price of each car - work it from the angle that "I'm going to give you a set of keys and a cheque - let's figure out how big that cheque is going to be". I've never cared one way or the other what my trade-in value is, just so long as the total transaction cost is where I want it to be.

Another question to ask yourself - do you really want someone looking to buy a $6K MB knowing where you live? Especially someone who found you through Craiglist? I'd rather not, if it was me.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)