I like the one that Carol mentioned but there are other possibilities for textbooks. Here's a list from University of Toronto Library of used bookstores with scholarly books: http://www.library.utoronto.ca/siteguide/faq-bookstores.html

It lists the U of T's own bookstore which stocks used texts. In addition, Ryerson University which is also downtown has a secondhand bookstore: http://www.ryesac.ca/services/life/usedbook.html

The main reference public library downtown has a used bookstore too: http://www.tpl.toronto.on.ca/sup_boo_index.jsp

Cheers,

Frances Toronto, Canada (in my other life, a librarian in the public library!)