I wish I could accept books to sign and return — I used to, but came to the realization I was spending 4-5 hours a day dealing with snail mail requests and signed books, and that it was actually starting to prevent me from writing. So I made an arrangement with Book Moon Books, an independent bookstore run by Kelly Link and Gavin Grant — I make sure they always have signed copies of my books on hand, and you can order any or all of the books by calling the store or ordering online. They have signed hardbacks, paperbacks, box sets, etc, and will ship anywhere within the U.S.
If you are outside the US and want a signed book, then you can contact Odyssey Bookshop and they can arrange that for you!