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.
