I'm the kind of guy that thinks I might be able to sell it for a good price, if it is high quality, after the use. Making the total money spent way less. But that begs the question, why not rent these out for 18 months at a time?
Also car seats can't be used after a crash, even if visually they looked ok. Maybe they could be refurbished (new foam, etc), but obviously this is a liability concern and probably isn't worth it.