If you are looking for cheap books, rare books, out of print books, books to sell or trade, then BMV is the place to go. It is a used-bookstore super store, with two floors of thousands upon thousands of titles and a multitude of genres. I've been to both the Bloor Street and Yonge Street locations and I'm equally happy with both.
BMV is a book lovers paradise. You will find what you are looking for...eventually. The store is HUGE and you will definitely get distracted about 300 times, if not more when searching for something.
It isn't a place you can go to with a specific mission in mind; unless that mission is to become so torn with how many books to put away because you've found so many to purchase....Regardless, BMV is one of Toronto's best book stores, hands down.