The simple act of not being brand new does not make a seat unsafe. All of my car seats are used, some for weeks, some for years. They're all still safe.
Used seats are NOT unsafe seats. Seats that have been in a wreck, improperly cared for, missing pieces that cannot be replaced or that the average person would not know to replace, are expired, have had the straps washed, etc are unsafe. If a seat has never been wrecked, is a few years old, has been properly cared for, all pieces and manual are present, then the seat is perfectly safe. It's the act of determining if all of those are true that is tricky.
I would not buy a seat from you (not trying to be snarky, trying to explain) because I know nothing about you, you have no history here, and I cannot determine, well, anything. I would buy a seat from chickabiddy because I know she knows her stuff, I trust she's not selling something unsafe, and I am pretty sure if she had been in a wreck she would have posted here about it! The fact that she is a CPST weighs very little into my decision. I am as likely to buy a seat from Morganthe, equally knowledgable and trustworthy without the fancy title. Or Andie, for that matter. On the flip side, though, there are a few CPSTs on here that I would NOT buy a seat from, as well as other 'regulars' that I would not feel comfortable doing it.
The "do you trust this person with your child's life" standard is, IMO, a bit overdone. I trust that if I dropped my kid on chickabiddy's door and told her that I was in dire straights and needed someone RIGHT NOW to take him, that he would be alive when I got back. But I don't know her well enough to trust her to baby sit him day in and day out and make sure she does everything just like we want her to (don't know her well enough to judge). But I do trust her with a car seat, because I would trust her to keep him alive in an emergency situation, which is what a car seat does.
I hope that, uh, maybe cleared that up for you. If not, forget I tried. And I just used chickabiddy as my main example because she had posted on this thread. And she can install a Radian in a Subaru with a seatbelt and that takes some mad skills.