Another way to look at it:
If the low estimate was 8000 (appx USD12500) and Holt's seller's commission is 20% then the seller would accept USD10,000. I don't know what the relationship between low estimate and reserve is at Holt's but the seller would probably have had a reserve of about 6000 if Holt's permitted that spread. That kind of reserve would mean that the seller would have accepted less than USD8,000.
If I was a lefty and the gun fit and if I wanted it I personally would pay about USD10,000. but not much, if any, more. I would be far more comfortable in the USD8-9K range.
But I wouldn't take an auction house's word for it (ANY auction house) and would want to examine it myself or have someone do it for me.
