Custer State Park Buffalo Auction
Each November, Custer State Park auctions between 200 and 500 head of live buffalo. Buyers and spectators from around the United States come to watch and participate in the annual auction. Buffalo are generally purchased to supplement an existing herd, to start a herd or to eat.
For the park, the sale helps keep the buffalo numbers in check, so there's enough grass to feed them. It is an on-going balance that changes with drought, plentiful moisture and other factors.
Details here: http://gfp.sd.gov/state-parks/directory/custer/events/bison-auction/default.aspx