If your stock has gone to a negative number, this has most likely due to two identical orders being placed through your site at the same time by two different customers. Whilst this seems wrong that your checkout system would let this happen, there are a number of factors that you must take in to account that make it necessary to allow the checkout to do this.
Your checkout system currently checks for product stock levels on the first load of the page and only removes the stock once a confirmed order has been placed. This means that customers can not add something to their basket and then leave it in there for a long period of time. This could mean that other customers are unable to buy this product. If the customer holding onto the product in their basket never buys it, then this will result in a loss of sales. We believe this to be a negative factor for both the seller and visitors to the site.
If you ever experience this occurrence, then we would recommend you update your stock back to 0 until you are able to re-stock the item. You should then also update the customer who completed the purchase at the later time and instruct the item is out of stock.