It is not, in the US, illegal to resell a car seat. As a technician, I certainly recommend that no one ever BUY a used car seat. If someone comes through a checkpoint with a car seat from Craigslist/Freecycle/yardsale/ebay, I confiscate it and replace it. It's an unknown source that does not meet the NHTSA guidelines for reuse. If you are going to reuse a seat, use one from a trusted friend or relative that you trust implicitly, and you know for a fact that
1. the seat is not expired
2. has never been in an accident
3. the harness has never been washed or exposed to harsh cleansers
4. has never been checked as baggage on a commercial airline
5. has never been mistreated
As an aside, AlainaFrederick, there is no First Years seat that goes to 40 lbs. The First Years convertible only goes to 35. It is rated from 5 lbs and with the newborn insert fits newborns fairly well, but the shoulder straps are fairly wide set and may provide a poor fit on some very young babies.
An infant seat is certainly not needed. It's a matter of preference and whether or not you find it convenient for your particular family. :-) I, personally, prefer them.