one's regular tradesman; regular supplier
Refers to a merchant or tradesperson who regularly visits a household or business to provide goods or services, often with a long-standing relationship.
Our regular supplier always brings us fresh vegetables.
出入り means 'coming and going' or 'frequenting'; 出入りの商人 specifically refers to a tradesperson who regularly visits.
商人 is a general term for 'merchant' or 'trader'; 出入りの商人 emphasizes the regular, visiting nature of the business relationship.
Compound of 出入り (でいり, 'coming and going; frequenting') + の (possessive particle) + 商人 (しょうにん, 'merchant'). The phrase literally means 'merchant of coming and going', referring to a tradesperson who regularly visits.