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.
