Are you trying to revise a saved listing in Auctiva or an active listing on ebay?
Did you list it as a fixed price listing (BIN only), or an auction with BIN (opening bid plus BIN price)? If it's a live listing in the fixed price auction format, I don't think ebay will let you change the format.
Starrynight's correct. If the listing is in the fixed price or store format, in order to make it an auction, you'll have to use the "Send to Auction" link after accessing your listing through your My eBay page.
If the listing is in the auction format with a BIN price, I believe you can remove the BIN by using eBay's "Revise Item" form.
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