I deal with both Play and Amazon on a work basis.
Play used to be head and shoulders above Amazon a few years back. Everything they had listed they would have in stock plus some.
That all changed when they bought in a new buying team, funnily enough some of the Amazon buying team. With the result Play trying to become like Amazon but at the same time cutting back on their purchasing and usually only ordering to order, something Amazon do not do. If Amazon have something listed then they will make sure that they will always have that item in stock, if Amazon do not have an item in stock, it's not down to Amazon, it's down to their suppliers.