Are you listing in a store or as a straight 7-day auction?
If auction....ebay requires a starting price...just make it the same as your Buy-it-Now or you can do a bit less, your choice - but they won't allow the field to be left blank.
You also have the option as selling in a Fixed Price format...this feature is in "auction" format and allows for you to list ONLY a Buy-it-Now price, which is probably what you're wanting to do...so fill in the FIXED PRICE circle at the top of the page when you're creating your auction and that will allow for a Buy-it-Now price only...a starting price will not be necessary.
I hope that helps
Good Luck,
Kathleen