I'm wondering because today I saw a few and sent a quick message saying something like
"Hi! Please check the expiry date on your car seat as this can compromise the safety of the seat. You can find an expiry on the seat, or you can call the manufacturer to find out whether or not it is expired. Thanks!"
There were just a few that were suspect, and one for sure that was expired. Seller even said it was expired and was still trying to sell it. Anyways.. someone I talked to said that this was really insulting and that its buyer beware. BUT at the same time someone emailed me back and said "Thanks! I didn't know that the seats expire that soon. I thought they lasted for 10 years. Good to know" type thing. So... I would think that personally I would much rather someone tell me privately that something I was selling was potentially unsafe, rather than sell it then realize that it was not safe.
Obviously some people don't care... but for those that can be clueless (like I usually am for many things in life) it might be a life saver... to me at least. Maybe it is insulting.