No is a great word that far too many of us don’t use when we should. I have no problem hearing or saying no. It’s a definitive answer. Most of the pain in the above posts could be solved with LTG’s “No”.
I’ve used it before and completed a sale. If the offer is too low, I usually say something like “Thanks for your interest, I’m going to pass on that and stick with my last price. Best of luck in your search”. I get to say no and still be polite. Win/win for me.