Great used book store! I hardly ever come in here, but most of their books are in immaculate condition. Tons of titles to choose from. If you are looking for popular title in particular, this is a good place to start. I managed to find a best-seller for $9.95 and it originally retailed for $20. They also have a cart outside, and from time to time you can find some really cool impulse reads for $1 or even a cool vintage magazine. The collection of DVDs looks impressive as well. I'm sure there are lots of gems among the shelves.
Most of the time they are open until 11pm, so if you happen to be strolling around this area, then this is a great place to come to on a walk home or on the way to the subway.
I have no idea of what it's like to sell books to these guys. In my experience with used book stores, it's quite difficult and you barely get any money (roughly 20% of the original price). Some places tend to be more picky than others. Based on the books in the store and based on the other yelp reviews, my sense is that ABC is probably really picky when buying books from people. On the other hand, as a purchaser I am impressed with the amount of titles and the condition they are in. I find their prices fair, but if you are still concerned, then check out BMV books just a block before Dundas and Yonge.