Take apart for storage or shipping, as in We need to knock down this chest to ship it safely overseas. [Mid-1900s]
Declare sold at an auction, as by striking a blow with a gavel. For example, That was the last bid, and the first edition was knocked down for only three hundred. [Mid-1700s]
Reduce the price of, as in They knocked it down by another hundred dollars, or An overabundant harvest will knock down corn prices. [Colloquial; mid-1800].
Earn as wages, as in She knocks down a hundred grand a year. [Slang; 1920s]
Steal, embezzle, as in He was caught knocking down the box-office receipts. This usage may be obsolescent. [Slang; mid-1800s] Also see knock over, def. 2.