You post your offer as a comment to the appropriate main-post, I do believe - and then post a link in whatever place might best serve to solicit bids.

Personally, I wouldn't sell a signed book if I liked the author a lot; you might not get another chance and, well, even if you bought it pre-signed, it's still A Book With A Story. (I'm very sentimental about those, myself, but of course you might not be.)