The ordinary condition words apply here, with three extra facts that do not appear in any other category.
The three extra facts
Date of manufacture, where the item carries one. Most safety-critical products do. Expiry, where there is one — car seats carry a manufacturer expiry date. Recall status, checked by you before listing, because a recalled item may not be sold. Prohibited items.
Then the ordinary ones
As new — unused or used once or twice, with tags or packaging. Gently used — used for a season, no faults worth naming. Well used — real wear, described.
Follow with specifics: stains and where, tears, fraying straps, cracked or crazed plastic, stiff mechanisms, wheel wear, fading, missing non-critical extras.
Completeness is condition here
List the parts. For anything that must be installed or fastened, "complete with all straps, buckles, fixings and the manual" is the sentence a buyer is looking for — and if something safety-critical is missing, the item is not for sale here at all rather than for sale cheaper.
Cleanliness is condition too
Say what you washed, with what, and what could not be cleaned. A stain that was described is a discount; one found in the parcel is a return and a rating.
History, for safety items
For a car seat, say whether you bought it new, whether it has been in any accident, and whether you can account for its whole life. "I bought it new and it has never been in a crash" is worth real money. "I bought it second-hand and cannot say" is honest and should be priced as such.
Questions people ask
Is a small stain worth mentioning?
Yes. In this category buyers look closely, and a mentioned stain costs far less than a discovered one.
What if there is no date on the item?
Say there is none you can find. Do not estimate an age you cannot support.
Can I describe something as sterilised?
Say exactly what you did instead. A process is checkable; a claim is not.
More: condition words across every category, buying baby items.