Second hand seats are fine to use provided we trust the person we’ve got them from. If buying from a second hand shop or market, don’t know the history of the product, also probably not get the full instructions either. Not worth the risk of using those. If getting them from a family friend or relative and trust them, no problem at all in using a second hand seat.
Make sure the seat is less than 10years old, this is a basic shelf life for this type of product. All seats today will have a date stamp on them showing the date of manufacture. Find the date stamp to ensure the child seat is within the 10 year shelf life.