Phil,
Thank you! I appreciate your comment. The 8.5 lb. weight seems too light to me as well. The seller says that a 10 pound dumbbell registers exactly at 10 lbs. on his bathroom scale, and makes his 8.5 estimate on that basis.
I don't think he's trying to mislead -- he offered to take the bass to a shipping store to get it weighed. It's just that bathroom scales are far more precise in higher ranges, e.g., in measuring the difference between, say 125 pounds and 133 pounds.
--Joe