Bookshop.org - A way to purchase books online while still supporting your local brick & mortar bookstore (US and UK only)

More info on their About page. But essentially, they take a small cut to cover operating expenses, and the rest of the profit from the book sale goes to whatever local bookshop you choose, as long as it's participating.

They also operate in the UK here.

They appear to be pretty legit, though one downside I've read is returns are more clumsy than other storefronts.