What evidence serves as Poirot's main focus in the murder investigation in the novel, The A.B.C. Murders?

Cust was tricked into selling the pantyhose where he sold them, and Poirot, when he saw this piece in every place, focused solely on it, and not on the other pieces in the case. Because Cust sold the pantyhose, he was wrongly accused of being the ABC murderer.