I just got my latest OUP royalties statement, and thought readers might find this interesting (or instructive or maybe depressing). The co-edited Nietzsche and Morality book (2007) that I did with Neil Sinhababu has sold over 1200 copies, since publication (805 hardcover, 450 paper, and 1 e-book). The old OUP Readings volume on Nietzsche I did with John Richardsonn back in 2001 has lifetime sales of nearly 2,400 (all paperback, no hardcover edition). The more recent Oxford Handbook of Continental Philosophy (edited with Michael Rosen, 2007) has sold an amazing 560 copies in hardcover (amazing given the price) and nearly 500 copies in paperback. What bears emphasizing is that these are good sales figures in academia. Nietzsche sells!
(My Routledge Nietzsche on Morality has sold around 5,000 copies since 2002, though I haven't seen recent sales data on it. So that's a regular "best-seller"!)