Maybe I'm just being ignorant here (I don't have any kids), but what's wrong with used car seats? Assuming, of course, they haven't been involved in a crash.
How do you know if it's been in a crash or not though? Even just being in a hot climate where the sun beat down and weakened the plastic could be enough to render it unsafe in a crash.
OK, are we talking about used things, or recalled things? Recalled products are probably not safe, regardless of whether you bought them new or used. I'm simply wondering what the big deal is about feeding your kid from a used (but sterilized, of course) baby bottle or laying him down in a used crib, or dressing him in used clothes?
I used recalls as one way of showing how a used product might be unsafe. However, that bottle you're talking about might have been microwaved every day for years leaving the plastic unsafe even though you're explicitly directed not to do this. It's hard to tell how well-maintained some things are, because even the well-maintained used stuff has been beaten on by kids for most of its lifetime and has the battle scars to prove it.
To me it's a question of what might happen if the product failed in it's intended use because of prior mistreatment. For a car seat, that's a really bad scenario. For a bottle from which my son ingests everything he gets, that's a really bad scenario. For clothes, the downside is nothing, so he has almost all used clothes. For toys that are really easily disinfected, the downside is nothing, so he has all used toys.