i've tried the nice and polite... it only opens the door to trouble. buyers, especially, feel they can then start asking/demanding stuff and many sales have actually been halted the friendlier you get w/someone and they want conversation and additional questions and etc. i'm polite on ebay, where you have to be, lol. it's not weird to ask what's wrong, but the buyer could provide a phony reason to you anyway... you may/may not get the truth so not sure how that's helpful
well if you see that buyer / seller is not friendly in their answers, you can always stop the conversation. though I recently had one, who accused that I sold her fake. I know I didn't and Tradesy agreed the item is authentic, but that buyer had her own opinion. she wrote smth like "you know exactly what you sold me and I want you to know what I think of such sellers as you" blah blah blah. that was very offensive and in that case I regretted that I asked first what was wrong.
but all others were super nice.