To whom do we owe the truth?
Everyone, but especially God. Whenever we lie, we offend God.
For instance, if the Fuller Brush man comes to my door, ( I know I'm dating myself here ), and I tell my kid to inform him that mommy is not available, when in fact, I am, is this an immoral act?
Yes, it is immoral to tell a child to assert that a falsehood is true.
Do I owe this man my time and am I obligated to listen to his sales pitch so as to avoid a lie?
You are obligated to refrain from lying; you owe this obligation to God.
You are not obligated to listen to a sales pitch.
You can say, "I don't want to listen to your sales pitch."
In other words, am I to engage in conversation with this man just because he wants me to?
No, you are not obligated.
Or, do I act rudely, and tell him that he is not welcome at my doorstep selling his products?
When acting rudely is not a sin, then you may act rudely.
It is not a sin to tell him that he is not welcome to sell at your doorstep.
It is not moral to lie in order to be nice.
Roman Catholic theologian