Some people are just ashats. Check his feedback left for others and see if this is a regular thing. You could try sending a request to review the feedback to the buyer, but as he's not replying to your other messages, it's unlikely he'll respond to that. You could also leave a reply to his stupid feedback message.....but keep it civil. Something like "buyer left ambiguous feedback without contact. Item new as described" (adjust so it fits into the 80 characters if need be). It's unlikely 1 red dot will affect your sales, especially as the feedback was ambigous. I buy from plenty of sellers with red dots and have never had any issues....but I always see what the red dots are first. Most of the time they are ridiculous comments, like 'outside of envelope was dirty'. One even complained because the stamp was crooked on the envelope (seriously!!!)! Some people are just keyboard warriors and love the anonymity of the screen to hide behind and throw their weight around. Seriously, I doubt that comment will affect sales and I think anyone who looked at it before buying would feel the same. If it does turn someone off, then you probably don't want them as a buyer anyway.
The issue with the buyer could have been something as simple as the measurements being in inches instead of metric (I'm guessing the measurements on the backpack are probably in inches?). Some people really are that pathetic. Don't forget to add him to your blocked buyers list.....and don't leave him any feedback.
