What does the boss mean when he says, "Say what are you sellin'?"?

The boss of the ranch finds Lennie and George's relationship unusual because they travel together. George takes care of Lenny and Lenny puts his trust in George. This is not a normal situation among "itinerant" workers during this time. The boss actually thinks that George is taking advantage of Lenny.That is why he asks, "Say what are you sellin?".

What does the boss mean when he says to George, "Say-what are you sellin"

Why does George lie to the boss about the bus driver