Minor store bug

There is a minor bug in store.

If you try to buy 1 Greater Mark of Magic Penetration, the item is on sale. If you want to buy 2, the marks are without sale. And if you go -1, there is no sale anymore

The bug is shown in the screenshots below:

1- http://imageshack.com/a/img924/1825/dcNxWc.png

2- http://imageshack.com/a/img923/7122/2eDiUN.png

3- http://imageshack.com/a/img921/2686/Fhj6Dy.png