I think the number of people oblivious to the fact that stuff like helmets and car seats have the hazard of being compromised from a crash is low enough to not distort the market much. That's part of why these items have such a discount used. Meanwhile you can sell stuff like durable name brand tools commonly for near new price.

Unfortunately when a new child arrives finances tend to be at a minimum and this deep discount works out as a almost always better than nothing and for super cheap.