Regarding this is subject what The Royal Prevention Society for the Prevention of Accidents (RoSPA) recommends:

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The best advice is, do not buy a second-hand child seat.

You cannot be certain of its history.

It may have been involved in an accident and the damage may not be visible.

Very often the instructions are missing from second-hand seats which makes it more difficult to be sure that you are fitting and using it correctly.

Second-hand seats are also likely to be older, to have suffered more wear and tear and may not be designed to current safety standards.