I dump them, because we are selling so many "new" toilets. I don't know what others do. I have had a few people over the years ask on the phone about buying a good "used" toilet.
It's pretty funny really, because when people have spent good money for the replacement, you can imagine how bad the old one was. They have been glad to be rid of them.
The first thing I did when I moved in here, was to pull all the old toilets. And frankly I remember dating a woman, and after a bit I asked if I could replace hers. Actually, I have done that before. It must be a habit of mine. I'm so used to installing new stuff, that I do it in my spare time even.