Absolute Certainty is the state that sellers get into when attempting to convince someone to buy. The idea is that when contemplating a purchase, people fall somewhere on a scale between certain and uncertain, and that potential buyers are more likely to be convinced by someone with complete conviction.
This technique was popularised in the finance industry, (clip from Boiler Room) but an interesting use case is how Donald Trump will announce his dominance and brilliance (proof) in areas where he lacks expertise. The net effect is that a certain percentage of the population are convinced by his conviction.
Don't con people, you lose money long term. But when trying to sell something you believe in, sell it with conviction. When having to sell repeatedly, take a note of peoples arguments against your point of view and formulate convincing counter-arguments.
On the flip-side, be aware of people trying to perform this technique on you, and do your own research when making big financial decisions.