The Pilot said:
I wouldn't buy used, don't know the background, or even if the seat is counterfeit. When I had our car seat inspected (90% of car seats are installed incorrectly at least in CO) the inspector mentioned he's seen an uptick in counterfeit car seats the last couple of years.
Ah yes, the temu effect. Get a Doona for 50 bucks. No way it's too good to be true! Will disintegrate in a collision.
Avoid used unless it's someone you trust with that child's life. Each seat has very specific care instructions laid out in the manual and just like most seats are installed incorrectly, most folks have no idea there are care instructions. Hose the seat down after it was vomited in? You have now ruined the seat. No bueno for the internal parts to get all rusty. But very few people realize this or would even mention it when selling or giving away.
The absolute key to whatever seat you end up with is to read the manual inside and out and install it correctly. There are strict use rules about things like space required between top of head and top of seat, and strap location based on rear or forward facing. These rules are based on general crash test physics and crash test results with that specific seat.
As for picking a seat, ask the parents first. If they're like me they have strong preferences. Then if it's a seat likely to be uninstalled frequently and used between kids, ease of install and adjusting kid to kid should be priority after whether or not the seat works well in your particular vehicle. Csftl.org and safeintheseat are great resources for this kind of research. You can also schedule an appointment with a cspt to make sure it's installed and used correctly. Many seat manufacturers also offer this service remotely.