Walmart sells junk because the marginal cost of junk physical goods is lower than the marginal cost of quality physical goods.<p>In contrast, the marginal cost of an ebook is zero. The marginal cost of a paper book is independent of the quality of the book.<p>A book makes money on volume (enough volume to cover fixed costs), not price. A junk book will simply not achieve enough volume to cover fixed costs, even in the Amazon marketplace.