How did Dions dad know that the coin dealer was dishonest?

Dion's dad suspected the coin dealer was dishonest because he noticed discrepancies in the dealer's pricing and the quality of the coins being offered. Additionally, his experience and knowledge of the coin market helped him identify signs of deception, such as overly aggressive sales tactics and vague answers to his questions. This combination of intuition and expertise led him to distrust the dealer's integrity.