From the setting page you can activate the "Donate Auction Items" button. This will place a button on your auction page so visitors can donate auction items online. Every item donated online will need to be approved by you before they will show in your auction.
1) Click "Auctions" on the far left then Click on "Manage Auctions".
2) Click on the Name of your auction.
3) Click "Design".
4) Click "Page Features".
5) Check the box next to "Donate Items".