The 22-year-old granddaughter of Queen Elizabeth II put the Philip Treacy creation on sale to raise money for UNICEF and Children in Crisis.
The silk hat caused a stir at the wedding of Prince William and Kate Middleton on April 29.
Since then, it has been compared to antlers and a pretzel, and has been photoshopped into scores of unlikely scenarios on the Internet. Forty bidders vied for the prize, but eBay did not reveal the identity of the winner.