You can't necessarily see damage from a crash even if the seat is there in person. I'd consider buying one in swap from one of the trusted members of this board. I can't imagine they'd steer you in the wrong direction.
Right. It's highly discouraged to ever use a Craigslist seat -- You have absolutely no idea on the history of the seat. People can say anything about a seat, like you said, but you can't always see the damage and you can never know for certain that a seat hasn't been damaged/crashed/etc.
People can (and DO!) lie, just to get rid of things and to (sadly) make a buck or two. They don't realize how deadly important a lie can be when it pertains to a car seat.
Would you trust that stranger's word with your child's life? I wouldn't. I probably wouldn't even trust a friend's word with my child's life.
There's no way to be 100% certain on the history of the seat, unless you buy new. And there's no way I would ever use a seat from a complete stranger. I don't even buy from certain members of this board, simply because I do not know them well.
As far as the car seats you children are riding in now - Were they previously owned before, too? If so, I would replace them right away. Like another poster said, if you like the Regents, you will likely like the Frontier 85 (not the Frontier 80 - They are different!). I love mine!
Here is the link to a nice, thorough review of the Frontier 85 (FR85), done by our very Admin, Darren.
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But in all honesty, I wouldn't drive with your kids anywhere in an unknown-history seat; it's that big of a deal.
ETA: The Regent's top slot is about 19.75", and the Frontier 85's top slot is right around 19.5", so as far as that is concerned, there isn't much difference.