This shop has a small window of five minutes in which you can complete checkout. When you proceed to checkout, the items are put on hold for five minutes to give you a chance to pay. During this five minutes, another customer is unable to pay for them which should prevent the occurrence of overselling. Items in your cart are NOT yours until you proceed to checkout so you may find that items disappear from your cart when sales are fast moving. If you find that you're unable to add an item to your basket, this will be because somebody has it in their basket and has proceeded to checkout. Stock numbers are not altered until payment has been made.
I cannot promise that this new cart hold system with be completely perfect but I do hope that it will limit the number of items that are oversold. If on the rare chance that it happens, I shall refund you in full for the item.