Just wanted to echo what others have said-
After we were in an accident I demolished the carseat straps before I put them out for trash and then I covered them with garbage bags. The covers I freecycled and some of them I sold on Craigslist ($100 for two of them, they were expensive carseats.)
I have purchased carseats at resale shops, received them on freecycle, and the last one I purchased on craigslist. My sister is using it now. Some consignment shops have them tested to ensure that they are sound (my sister said it looks for minute stress fractures ect.)
The resale shops around here sell them all the time, but I don't know if I've seen them at Goodwill.
S.