Well, I donate old clothes to charity drop-off locations and I donate books to libraries, but it doesn't come up very often. If I can, I will, though.
As far as buying used stuff, well, I went to a used book sale for a charity today. I guess that counts. I got a ton of books for like forty bucks, a lot of books were great deals. I got an old Merck Veterinary Manual for like four bucks, a book on Albrecht Durer for another four bucks, some more comic books for my "bathroom library" (now I've got two shelves of books in my bathroom), a half-dozen books on Biology and one history book. Oh, and a cookbook. And the book that inspired this list. If there were more used book sales, I'd be going more often.