As I see it, they're both charging 15%. (Play just does it as two separate transactions, at 10% and 5%). But Amazon want 75p + 15p (VAT) + 43p on top (= £1.33), whereas Play only wants 50p. So that's a saving of 83p by using Play. (It might actually be slightly greater, because I've not factored in Amazon charging commission on p&p).
However, if I was looking for secondhand books, the first place which I'd look would be here:
http://www.abebooks.co.uk/
(although that site is really only useful if you're selling large quantities of books), and then Amazon. It would never even occur to me to look at Play. So they might be cheaper but you're probably less likely to get a sale on their site.
Chris