I wouldn't care about an expiration date, but I avoid the hand-me-down car seat market for the wreck reason. The carseats are only designed to be in one. I personally wouldn't know how to definitively say it had never been in one.
Hand-me-down, to me, connotes reuse within the same (possibly extended) family.
I'm fine sharing our used carseat with my 6yo only child's grandparents so that they can more easily help with my nieces and nephews (2 weeks, 1yo, 3yo, 4yo, and 5yo - oof!). My wife and I know it's not been in an accident, we would not misrepresent that to the detriment of our own family.
But I would not buy one from even the most trustworthy Craigslist or Facebook Marketplace listing imaginable.
I personally would love tamper-evident components within a carseat - think "Tip and Tell" [1] but for 3-axis accelerations. Impact-sensitive product labels exist such as those at [2], but I'm not convinced that the same accelerations and crashes that would damage polystyrene impact-absorbing foam would set off a glass ampule designed to break when you drop a rental camera lens or something like that.
