None. The publisher gets money from the bookstore when it originally buys the books from the publisher, after that it's property and the used bookstore is just re-selling it with nothing going to the publisher. Now, the original sale/stocking can get a little fuzzy, especially because these days publishers and bookstores have weird deals/fall under the same corporate umbrella (for example, B&N does do some of it's own printing for the "classics", i.e. stuff out of copyright).