Depends on how old it is. I read or was told that baby seats have a 6yr. life span because of strict guidelines and safety updates.
If it's under 6 yrs, I'd give it away or donate it to the Diabetes Foundation (pick up old furniture, clothes, etc).
Here is a site that lists expiries by manufacturer:
UPDATE - Children?s car seats and booster seats: How long are they safe? - Transport Canada
That being said many organizations are starting to disallow the selling of used car seats (craigslist, kajiji) or accepting donations due to liability issues.