I wouldn’t sign nor sell a used seat. However, many charities will take used car seats, and will have car seat technicians check and recertify they as being said. However, they will not recertify for it to be resold, but only to give to charity. I asked why that was, and they said if a car seat is sold for money there is a higher liability. To me, it wasn’t worth it, but I hated throwing the great seats out,s I always donate.