On Feb. 12 last year, the same day that a cease-fire ended the worst of the fighting in eastern Ukraine between rebels and government forces, a former rebel fighter seized a chance to turn his inside knowledge of the conflict into hard cash.

He traveled to a spot on the Russian-Ukrainian border where he retrieved a cache of weapons hidden there earlier by his comrades in the pro-Russian rebel movement.

Four days later, shortly before 6 p.m., he and a friend showed up in a taxi at a fuel station in western Russia where they had arranged to meet a contact ready to buy the arms, according to Russian court documents.