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Low compression in diesel mercedes

I’m looking into buying a diesel mercedes and the seller says it has low compression, so sluggish to start, but runs great. Is this a red flag? It seems to be the only drawback. Opinions?

Engine compression is critical, especially in diesels. I wouldn’t buy it until the cause is identified and solved, if it can be solved. It might just need a valve adjustment. Or the engine could be shot.

What year and model? How many miles?