O.P.
You might try ebay. It's really hard to sell car seats as most places won't take them. There's a store called Once Upon A Child that takes gently used children's items from toys to clothing and shoes, etc. I had a car seat that my son had outgrown and it was still in great shape but they wouldn't take it. You might want to go online, type in their name, look for a location near you and their phone number and ask them before you go down there with it. Now they will take strollers, depending on how old they are.... or.... garage sale.
Good luck to you!