What separates the straight shooter and the liar from the bullshitter? The relevance of what they believe to be true to what they are saying.
The straight shooter tries to communicate a true belief. The liar constructs a lie based on their true belief as well.
The bullshitter communicates without concern for truth or falsehood.
Here is a perfect example of the platonic Frankfurtian bullshit artist, Donald Trump, at work:
Mr. Trump liked to tell guests that the nursery rhyme-themed tiles in the room were made by a young Walt Disney.
“You don’t like that, do you?” Mr. Trump would say when he caught Mr. Senecal rolling his eyes. The house historian would protest that it was not true.
“Who cares?” Mr. Trump would respond with a laugh.A weaker example (but with higher stakes):
Ted Cruz can be easily attacked without resorting to bullshit. Yet on the day of the Indiana primary, Trump maintained that Ted Cruz's father was "with Lee Harvey Oswald shortly before the death, before the shooting" of JFK. This is, to put it mildly, an unsubstantiated claim. Trump doesn't know whether it's true or false. And he doesn't care.
I encourage anyone reading this to check out On Bullshit. It's a short and entertaining read and has helped me understand present-day problems with discourse.