Have you tested out buying some common items? Add some items that are common and already in the shop and see if it purchases.
Usually when it said 'Failed to purchase' it means it got sold out and you were too slow. But it may be some other error so do some trial and error with regular items before the rare ones and let us know.
And also, the thread in which you should be posting your troubles should be here: (you need an account to see links)