What should a person do with the cigarettes he has left after quitting smoking; should he destroy them, or give them to another smoker, or sell them to benefit from their price?

All praise is due to Allah for your repentance from smoking, and we ask Him to keep you steadfast in it. As for the remaining cigarettes, if it is not possible to return them for a refund, then destroy them, as they cannot be used for anything permissible. Do not give them to a smoker, as that would be assisting in evil and harming his health and his religion. And Allah knows best.