There is no grading scale here, and there are three facts that outrank condition entirely.
The three that come first
Recall status. A recalled item may not be sold. Look up the model before listing. Prohibited items. Completeness of anything safety-critical. A missing strap, buckle, pin or fixing means the item may not be sold — not that it is cheaper. Dates. Date of manufacture where the item carries one, and expiry where there is one. A car seat carries a manufacturer expiry.
Only after those three does condition mean anything.
Then the ordinary words
As new — unused or used once or twice. Gently used — used for a season, no faults worth naming. Well used — real wear, described.
Followed by specifics: stains and where, tears, fraying straps, cracked or crazed plastic, stiff mechanisms, wheel wear, fading, missing non-critical extras. Describing baby items.
Completeness is condition here
List the parts. "Complete with all straps, buckles, fixings and the manual" is the sentence buyers are looking for.
Cleanliness is condition too
Say what you washed, with what, and what could not be cleaned. In this category buyers look closely and reward it.
History, for safety items
"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 prices accordingly.
Questions people ask
Can I grade a car seat as "like new"?
Only alongside the expiry date, completeness and its history. Those are what the buyer is assessing.
Is a small stain worth mentioning?
Always. A mentioned stain costs far less than a discovered one.
What if there is no date stamp?
Say there is none you can find. Do not estimate an age you cannot support.
More: buying baby items, what they are worth.