You don't give a conman your confidence, he gives you his.

I saw a clip of a film some time ago (anyone idea of its name?). It has Chazz Palminteri in it. He played a conman. Him and a woman were waiting in a Western union (?) office. They got chatting, and in walks a GI. Chazz's character talks to the GI and starts to con him out of his money. The woman was aware of this. But he stops short of conning the GI. The GI leaves, and he explains how the con works.

Chazz and the woman then leave together. The point is that he was playing a long con against the woman and the aborted short con was just a ploy.

The moral of the lesson is that if someone reveals themselves to be a liar, then you should take them at their word. Having established themselves as liars, you should never trust them.