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?
renting assumes previous users properly maintained these seats. Car seats are "supposed" to be disposed of in the event of a collision due to possible cracks or other fatigues in the structure not necessarily visible to the end user. If you rent, you have to assume the seat was not in a structurally significant event. That's a lot of trust.
The foam doesn't stay good for long, I think the car seats we had expired after 6-7 years?
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.
Most likely because you can't guarantee how it has been used once the first customer has used it. And that will lead to major legal problems if e.g. it has been in a crash and is now compromised, but nobody noticed, then failed to protect the 2nd child.
