I bought a '92 300e 2.6 on ebay about a year ago and it has been a wonderful car. I made sure to search just the Chicago area when I bid so I would be able to look at the car before I bought it.
The car had 106,000 miles on the 2.6 engine (same engine as the 190e 2.6) and runs beautifully. The only problem I had was that the engine used quite a bit of oil (common for these engines). I replaced the valve stem seals myself for less than $20 and oil consumption went from about 1 quart in 1000 miles to less than 1/2 quart in 3000 miles.
As far as I'm concerned, condition is the main factor in determining a price for a car, especially a ten year old one. You might find a car $1,000 below book value but end up spending $2,000 in repairs the first year. Service records, number of owners and general care and condition should tell a lot about how well a car has been maintained. There are plenty of 10 year old Mercedes cars out there that have been very well kept and are in near mint condition.