Audrey Tracy does a great job of answering this question in the eBay for Business blog…
Fixed price is the right format most of the time. Ask yourself if:
You know the value of your item
You know the exact price you need to get for the item
You have more than one to sell
You have a lot of items to list and want to minimize the cost of listing
You want a listing to last longer than 10 days
You—re willing to negotiate, using the Best Offer option
Auction format is the best option if you have only one of an item and don—t know for how much it might sell. Make sure you check Sold listings on eBay, or use Terapeak or Worthpoint, to help determine its value.
Read more (and subscribe to eBay for Business) at http://for-business.ebay.com.