Buying used for children is ordinary and sensible. It has a short checklist, and it is worth doing every time.
The questions
"What is the model number and the date of manufacture?" Shown on camera. With both you can look up the recall history yourself — the one check nobody else can do for you. Non-answer: "It's a well-known brand."
"Can you lay out every strap, buckle, fixing and insert?" A missing safety part means the item should not be on sale at all, so this is not a haggling point. It is a stop. Prohibited items.
For a car seat: "What is the expiry date, and has it ever been in a crash?" Seats carry a manufacturer expiry. A seat that has been in a crash should not be reused, even where nothing shows.
"Do you know its full history?" "I bought it new and it has never been in an accident" is worth real money. "I bought it second-hand and cannot say" is honest, and should be priced as such.
"Has it been washed, and what could not be cleaned?" Buyers here look closely and sellers who answer well are the ones to buy from.
"Can you work the folding mechanism on camera?"
What protection covers
A recalled item, a missing safety part not disclosed, an expired seat sold without its date, damage not shown: seven days from delivery. A recalled item should not be listed at all — report it.
Questions people ask
Should I buy a car seat second-hand?
Many do and many will not. The checks above are what make it a decision rather than a gamble.
What if there is no date stamp?
Then its age cannot be established. Pay less or pass.
Is the manual important?
For anything that must be installed correctly, yes.
More: buying baby items, what they are worth.