1) Click on "Manage Inventory" under the 'Inventory' heading of the Control Panel.
2) Search for your item by it's Item ID (or Author / Title)
3) Scroll down to the 'Search Results' and click the 'Edit' link under your item.
4) Click on the 'eBay Properties' button.
5) Scroll down to the 'Stock' heading & limit the 'Quantity per buyer'.
The 'Quantity per buyer' value can't be larger than the quantity being sent to eBay
6) When you are finished, be sure to click the 'Save Properties' button to finalize.