This looks like a Marbles sight, which was often substituted for the original. It probably shoots to point of aim and you can always replace it with an original fairly cheaply. A fair bore does not seem to affect accuracy in most people's experience. Chamber pitting may be slightly more of a problem, but would usually not affect overall function.
In 9mm it is worth the $750, although I would push for $650-$700 due to the "poor bore and chamber".
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