E.T.
Around here there's no resale market for carseats since they expire, and many charities will not take them for the same reason (and because you can never tell if a carseat has been in an accident). How about calling a women's shelter or homeless shelter to see if they could use a carseat?
That's what I did with my infant ones a few years ago. The shelter was thrilled to have them... and I didn't have the hassle of trying to recycle or dump them.