In my search earlier this year for a big flagship Euro car, I learned much. In general, the smaller MB cars are to be avoided. The bigger ones too, but that's a different thread :)
For $6k, you can buy an S-class, so I wouldn't even think about a CLK at 7500. $4k, maybe.
My specialty is finding diamonds in the pile of coal. That is to say, the whole lines of luxo Euro cars like MB, Jag, BMW, etc depreciate like a cannonball in a lake. The secret is to find a model that is actually a good car but suffers from its sisters' depreciation. Early S-class is a good bet. W210 E-class is a winner. Pretty much none of the C-class, M-class, or CLK fit that bill.
My initial reaction to that particular car is that it fits neither of the criteria: Its not one of the good bets, and they price they're asking isn't properly depreciated. I wouldn't even test drive it until they lower the price to $6000, and then I'd offer $4500.
Then expect to buy a $400 battery every couple years, $150 brake rotors every 1.5 years, and don't be surprised if the oil cooler o-rings blow out and the motor seizes about the same time the "low oil" light comes on.
I'm being apocalyptic and snarky, but the CLK is not one of MB's best. The rampant depreciation on these cars means you can shop around and find them cheap enough to cover the rampant problems on some, or use the depreciation of whole brand across the board and buy a rock solid model instead. If it weren't for the dated styling, I would buy another W210 E-class in a heartbeat, although their excellent track record is starting to bring prices back up.
Edit: Just did some checking on the local CL. A CLS55s for $7500, a 2003 E500 for $6000 with 100k, a clean-title 2006 S-class with 130k for $4000, and even an 01 loaded CLK at a dealer for $4k with 120k. Long story short, you can get the same car from a used lot for $4k. Like I said, depreciation is your friend if you find a model that isn't a turd.
You asked if it was too good to be true. I say he's asking double what its worth.