For instance the site is, right now, taking pre-orders, not orders. There is no published timeframe when they'll actually ship the books that I can see. Paypal only allows you to accept pre-orders up to 20 days in advance of shipping the product to buyers, and even then they can demand significant additional proof.
Because, unfortunately, pre-orders are the classic setup of too many scams (which some random agent probably won't easily be able to eliminate), and it's the domain where PayPal ends up holding the bag. 5000 people pre-ordered some cool internet controller and then maker disappears, etc.
We've been hearing these sorts of "horror" stories for years (wow do people not understand how hard and exclusive the payment process used to be!), yet despite PayPal still being relatively small -- especially compared to the banks and credit card companies -- no one is credibly doing what they do. Maybe because they're doing something rather hard?