The following factsheet is from Britain but of course it is valid anywhere in the world. It was aimed at the managers of charity shops that sell used items, hence the wording, but it applies equally to any purchase.
Sale of Second-hand Child Car Seats
We all know how expensive it is to bring up children and Charity Shops are a valuable way for parents to shop around for second-hand items, to save money whilst raising valuable funds for good causes. But, one item you should never take a gamble on [buying] is a second-hand child’s car seat.
What’s the danger?
It’s impossible to know for certain if a used car seat has been in a collision and relying on a [seller’s] word is too much of a risk to take.
If a car seat has been involved in a crash there may be little or no visible damage to it, but there could well be substantial internal damage, rendering it dangerous.
Missing instruction booklets in second-hand car seats could lead to them being fitted incorrectly, rendering them dangerous in the event of a crash.
It is also vitally important that the size of the car seat is correct for the size and weight of the child and, sadly, not all child car seats fit safely into all cars. Retailers of new seats receive industry training on appropriate restraints and can advise their customers on fitting them.
How can you tell if it’s safe?
With second-hand seats, you can’t. Just looking at a car seat won’t tell you what you need to know. In fact, the only way of checking a used car seat’s integrity is through laboratory testing.
The fact that many seats ‘look’ OK after an incident leads some parents to continue using their car seats after a crash, unwittingly putting their children at risk.
The Royal Society for the Prevention of Accidents (RoSPA) advises that car crashes can weaken a child car seat to the extent that a child is left dangerously unprotected in the event of another incident. They advise parents to replace their children’s car seats immediately after a crash.
How can you help keep children safe?
Please do not [buy or] sell second-hand child car seats… the consequences of a seat failing, even in a relatively low speed impact, are simply a risk not worth taking.
Source: http://londonroadsafetycouncil.org.uk/highlighting-the-dangers-of-second-hand-child-car-seats-to-local-communities/ on June 29, 2017
We would also add the reminder that PARENTS SHOULD **ALWAYS** SEEK EXPERT HELP IN FITTING CHILD SEATS CORRECTLY. AROUND 90% OF ALL CHILD CAR SEATS IN AMERICA ARE WRONGLY FITTED, AND THAT IS DANGEROUS TO YOUR CHILD! YOUR LOCAL FIREHOUSE / FIRE STATION IS A GOOD PLACE TO ENQUIRE, AS FIRE CREWS HAVE MANY TRAINED SEAT FITTERS, THROUGHOUT THE USA (and it’s usually free).