It is impossible to know for certain if a used car seat has been in an accident. Relying on a seller’s word is too much of a risk to take and the one item you should never take this gamble on is a child’s car seat.
If a car seat has been involved in an accident, there may be little or no visible damage to it. However, there could well be substantial internal damage, rendering it dangerous. The only way of checking a used car seat’s integrity is through laboratory testing. Avoid buying second-hand car seats if you can.