My first problem w/ ebay came up... needs your advices

I normally respond to the buyer withing 12 hours, that allows for times when I'm on an onsite job or my alloted 4 hours for sleep. I always try to stay on top of the auctions but I've had sellers repsond to me as buyer as late as 5 days but they did have a good explanation (in the hospital or moving). I have had only one little "dork" (for lack of a better word) as a buyer post negative on me that he got junk when I actually sent him a better unit than was posted. I've heard he's done this to others too and we've all come to the conclusion that he's got junk around and buys good units cheap, then wants a refund on a phoney charge that it's not "as advertised" and he sends the crappy one he had before the auction back to the buyer. I didn't go for it as I openly post "sales final" on my auctions as I'm not a friggin' retail store. Even at that this "polbit" character posts negative but with over 155 excellent posts I don' think one will hurt, especially since it's already buried deep in the rest of the feedback. He posted "buyer beware" when it's actually "seller beware of buyer". All in a lifetime I guess.