This paper presents a model of bidding behavior in multi-unit ascending (English) auctions with a buyer's option. The buyer's option gives the winner of an auction the right to purchase any number of units at the winning price. We derive the optimal strategies for the bidders and the corresponding expected revenue for the seller. The model is estimated using a unique data set on wine auctions held at Richelieu-Drouot. A counterfactual comparison shows that the seller's revenue increases by 7% if he decides to switch from a system where items are auctioned sequentially (i.e. one after the other) to a system based on the buyer's option.