There is opinion and there is demonstrated fact. The market may show occasional examples that one might argue are "exceptions to the rule" but that doesn't change the rule or make it any less true.

Aftermarket porting to the revolver reduces it's value without question and also makes it less marketable. However, to the right buyer, it could make it more attractive.

If you are the buyer and you like porting or you don't mind porting, then you are presented with a gun that has a flaw and you can use that aftermarket porting to aim for a lower price than offered.

If you are the buyer and you see an end game that has you possibly re-selling the revolver at any time in the future then go in to the purchase knowing full well that you are buying a gun that is less desirable than EXACTLY the same revolver without the porting.

I am happy to discuss further thoughts about .44 Magnum revolvers, porting, M-N-P specifically and I have hands-on experience with all three of those side bar topics but your post #5 makes it look like saying another word is unwelcome.