Using the reserve feature to an item can be done by entering a price value in the reserve field. This means that the item will not sell unless that reserve price is exceeded.
1) Click "Auctions" on the far left > Click on "Manage Auctions"
2) Click on the Name of your auction.
3) Click on "Items".
4) Click "Add Items".
5) Click "Yes" next to "Set a Reserve" and enter the reserve price.