As a grandparent with a very car seat-savvy child, they might have no value at all, if no one will buy them. Car seats aren't safe if they've been in an accident, have worn or stressed straps, or are older than a certain number of years (the plastic ages and becomes less strong). Since you can't verify any of that, what is a used car seat really worth?
As a tax person, the FMV is what a willing buyer will pay a willing seller in an open sale where both parties have fair knowledge of the transaction and no one is being forced or coerced. If you can't buy a used car seat from a thrift store or a Facebook marketplace ad, then it might just have no value. Have you checked at kids consignment shops like Once Upon a Child?
There's no standard percentage of value for FMV of used items. If audited, the IRS will want to see proof of what similar items actually sell for in your area, either from thrift stores or online sales or some other marketplace.
There is a user to user market. It's illegal to sell in stores or online. So there's a market but I don't know the pricing. That's why I asked. And, yes, I know all that other stuff.
Just think of what a reasonable purchaser of a second-hand seat might pay in determining the value.
Opus17 is right on target, and since you’re asking about an item that, as you say, isn’t legal to sell, coming up with a reasonable fair market value is particularly challenging.
That said, you might think along the lines of what the car seat might go for in a thrift store, or a garage sale. I’m a customer of both from time to time, and, depending on the shape the seat is in, you might fairly estimate an FMV of ten to thirty percent of what you bought it for. It should be roughly what a reasonable second-hand purchaser might pay.