From ebay:
Why did the Buy It Now option disappear after the first bid?
Buy It Now gives you the chance to purchase an item immediately, before bidding begins. But you have to act fast. Once someone bids, the Buy It Now option disappears. The listing then proceeds as a regular auction-style listing, with the item going to the highest bidder.
Special situations
In a listing with a Reserve Price, Buy It Now disappears as soon as the reserve price has been met.
In a few categories, the Buy It Now option may continue to be available after the first bid, but only for a limited time. Learn more.
and from the "learn more":
Update to Category and Price thresholds
The following categories will have Longer Lasting BIN w.e.f 10/17/2007 with a price threshold of 50%.
1. Parts & Accessories (eBay Motors)
2. Tickets
3. Clothing, Shoes & Accessories
4. Cell Phones & PDAs
Price Threshold: 50%
i.e. The BIN option will be available on an item till the current bid reaches or exceeds 50% of the BIN price.
Note: For reserve price auctions, the Buy it Now option will continue to be available till the reserve price is met. The price threshold is only applicable once the reserve has been met or exceeded.