Contrary to its recent misuse as a euphemism for audacity or courage, to have "brass balls" actually means to drive a hard bargain
and to refuse to sell for less than what you ask for.
The term dates back to the days when pawn shop
brokers would traditionally hang three brass balls outside their shop doors to indicate their services.
You mean to tell me that this guitar is only worth $15? You've got some real brass balls on you!