In many years of tracking auctions and locating benz, I cannot recall seeing a single example of a $6000 sale on that vintage model with this amount of mileage.
Value is dependant upon the overall condition, as well as the recentcy and detail level of maintenance and care, as well as well-documented repairs.
As a buyer, an average 300K mile car should be around $2500 to 3500 depending on repairs and maintenance that have been done, as well as understanding what repairs/replacements have NOT yet been accomplished. I have seen 130-150K mileage models in the bid ballpark of $6K, under 100K miles the bids are more like $6500 to 11,500...not positive the sales were ever actually completed... Got any pictures and more details? I'm sure folks on this site can help establish a valid approximate value.
Or you can post it for a best offer auction and let the market establish your sale price.
Good luck and regards,