When a crafter of goods fabricates an item of significantly higher quality, they may set it aside from the many other items that are simply "good enough" to send along to market. These high quality items can be referred to as "private stock" or "private reserve" and are usually either sold at a higher price, or reserved for special occasions. Incidentally, the items that were "good enough" might be referred to as "run of the mill" items.
The expression can also be used when the item wasn't crafted by the owner. For example, a wine collector may have a selection of common wines that he or she serves guests at parties, but may have "private stock" that they reserve for very special occasions.
In modern English, it is frequently encountered as a marketing device used to imply quality, often without actually having to make any quantifiable claims about the product. It's like saying your food contains "premium" ingredients. They are implying that their ingredients are better than the "standard" ingredients, but we have no way of knowing what makes them better.