After playing an instrument for years, the finger board need to be planed to take out dips and string grooves. If an instrument buzzes only on the open strings most likely the nut is too low. There needs to be a small amount of  swoop in the fingerboard. Too little and the strings will buzz, too much and the string is too hard to press down.

There is a shot of a fingerboard that was buzzing on open string. I have never seen one with such deep string grooves.

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