I try to buy most of my games new, but for some that I don't mind buying used, Glyde is the best option. They usually offer the cheapest prices for games which is really cool because it's driven by the market.
I enjoy the fact that when I'm selling, it's hassle-free. You don't have to worry about shipping your games because of the convenient prepaid envelope they send.
I received a different game from the one I purchased once, and the customer service was quick to remedy the situation and refunded my order.