Die casting process with a steel mold eliminates casting porosity to a minimum.
This is not correct. The die or sand mold design as well as the pouring technique it what prevents porosity.
A CNC machined cast iron impeller? Curious that they would machine them out of cast iron.
It is also odd that the person who's auction this is didn't take a picture of the business end of the pump to show.
I have no familiarity with this pump but I do working with water pumps, CNC machining, and some casting. Those are just 3 red flags that popped up for me.