I belong to a neighborhood buy/sell page on Facebook. Someone posted a couple of Britax marathons purchased in 2008 and 2009 (likely manufactured around 2007/2008). I sent her PM explaining that those seats have a 6 expiration date from when they were manufactured. She responded that she had no idea that car seats expire and took the post down. I was nervous about speaking up, but she was really nice about it and thanked me.
Good for you I was at a garage sale a few weeks back and they had a booster seat for sale. This man picked it up and then said 'oh it's expired' and the lady selling it said 'it was used 3 times' and I said 'I think the plastic wears out. Nothing to do with how much you use it unfortunately' She didn't remove it from sale but at least the guy didn't buy it.