Fifth is #WooCommerce, a WordPress plugin
Difficulty: Moderate to difficult, depending on your comfort with WP
Pros: Easy to install *if* you’re a WP expert, buyers don’t have to leave your WordPress site to buy your book, low transaction fees
Cons: Requires a WordPress site and hosting, more issues than other sales platforms
Cost: 2.9% + $0.30 per transaction in the US + the cost of your WordPress site