Honestly, I still wouldn't buy a second-hand one. You just don't know if it's up to the job and one of the handful of times it will be used might be the time that it is required to save your child's life. A car seat that looks perfectly fine could have sun-damaged, brittle plastic, a strap that will snap in an accident, compressed foam in the side-rests. Just not worth it IMO.
From BBC website:
Despite tempting cut-price offers, experts advise against buying second hand car seats.
To find out why, Inside Out puts a second hand seat to the test.
The £15 seat is tested in one of the most advanced crash test rigs in the world.
The results serve as a stark warning about the dangers these seats can pose.
The harness is clearly not strong enough and the dummy is thrown forward in the seat.
Brian Mycock of Trading Standards explains that the crotch strap would have caused considerable damage, whilst the dummy's head is forced forwards onto the dashboard, causing possible internal injuries.
"With second hand car seats you don't know how old it is, you don't know its history, or if it has been in an accident," explains Brian.
"The only time you should consider a second hand one is when you're buying from a member of family, whose knowledge and information you trust."