Final answer:
US Treasury bills are sold through primary dealers in collaboration with the Treasury Department but are not sold directly to retail investors; open market operations are employed to influence the federal funds rate.
Step-by-step explanation:
The sale of 3-month and 6-month US Treasury bills by the Federal Reserve involves primary dealers and is done in collaboration with the Treasury Department. However, they are not directly sold to retail investors as part of open market operations. The primary tool for conducting monetary policy in the U.S. is through open market operations, where the central bank buys or sells U.S. Treasury bonds to influence the quantity of bank reserves and the level of interest rates, specifically targeting the federal funds rate. It's important to note that open market operations are primarily used to control the supply of reserves that banks hold at the Federal Reserve, which in turn impacts the federal funds rate.