A Washington ivory and antiquities dealer will serve six months in federal prison after secretly pleading guilty to federal smuggling charges four years ago and then going undercover to help authorities prosecute a former Canadian Mountie described in court documents as “among the most prolific wildlife criminals ever prosecuted in this country.”

According to documents unsealed last week, David L. Boone, owner of Boone Trading Co. in Brinnon, Jefferson County, had been working with federal…