The Transformers star was a big spender at the Riders for Health auction on 11 July (09) when he placed bids on items including a $3,000 (£2,000) hand-painted racing jacket and a $2,500 (£1,665) helmet.
Proceeds from the auction will be donated to the international non-profit, which provides health-care to rural African villages using motorcycles.
A source tells America's Star magazine, "He was the talk of that party!".