If hardcopy, then eBay may be the way to go, although I never would have thought to look there for a cookbook.
If PDF or eBook, I think you might get better exposure and circulation using one of the ebook publishers. I love the instant gratification of ebooks and the fact that I can review a portion of the book before deciding to buy.
I just sold a copy of Fernand Point's book on Amazon in 6 hours for 100 bucks. Also sold Alice Medrich's brownie book fairly quickly. Then I start searching for some of the other 70's French cookbooks I have, Verge, Guerard, Troisgros, Bocuse, and they are dirt cheap. So I figure why bother? You can also sell books at abebooks.com.