Correct usage of phrasal verb [knock sb/sth down]

Question

I was doing a comprehension and I was to identify the adjective in a sentence.:

The piano was knocked down to the highest bidder.

I identified the adjective but could not understand the question correctly. Knock down is usually used as the cost was knocked down to £20

So how can piano be knocked down to someone?

Answer 1

To knock down has other definitions. This is the one that you need:

To declare sold at an auction, as by striking a blow with a gavel.