The recent appearance of the Italian branch of amazon.com is a boon to bibliophiles who want to obtain books from Italian publishers. Before now, there was the option of ordering books from individual publishers, but doing so involved difficulties (and in my experience, various tax and shipping issues that were not always consistently applied). I’ve conducted a few sample searches and the inventory at present appears to highlight more recent books, but perhaps the inventory will be expanded in time.
For used books in all European languages, search engines www.addall.com and www.bookfinder.com are quite helpful. One problem with the latter site, however, is that searches are categorized by language and there is no option for Latin books, so Latin books they are often lumped randomly with other languages. For specialized scholarly books in philosophy and theology, one should always keep in mind Loome’s Theological Bookstore, and (as has been noted) Henry Stachyra’s online bookstore. As its name suggests, the International Franciscan Bookshop holds many Franciscan-related items, including Quaracchi volumes.
Perhaps readers could make suggestions in the comments below for other outlets for new and used scholarly books. Unfortunately, amazon.fr has not kept up its stock of Leonine volumes. Where else might one turn?