Having been in the market to buy a 280 for the last 6-8 months I can pass on what I have looked at although I focused more on the 4.5 vs. a 6. Most notably was a 71 with a 6 early on for 3400.00 asking with prefect paint, interior, carpet, etc but needed some stuff like regulators, AC work, cooling help, etc. Later sold for 2800. There is a perfect low mileage sel 4.5 in S.D asking 6500 probably go for ~5500.

So far real nice ones seem to sell for 2500 to 3500 with ~150,000 miles.

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Litton
'90 420 SEL (sold)
'72 280 SEL 4.5
'98 ML320 (for sale)
'86 560SL
'05 Jeep Grand Cherokee Ltd (offroad in style)
'87 Chevy Blazer (AZ Pin Strips)