They put expiration dates on car seats because they say the plastic and Styrofoam start to break down after a time. Also, unless you know the person really well, you shouldn't except carseats because you don't know if they'be been in an accident. If you can't afford a new one, ask your Doc if there are any programs that will provide one. A lot of times police stations have programs like that.
Not smart. Don't use used
I would buy your own. Regulations on car seats change frequently which is why they say don't accept a hand me down car seat