Execution ballads were sold on busy streets, bridges, and marketplaces by street singers, who often bought the songsheets and pamphlets wholesale and then sang them as a form of advertising.
These sellers often lived an itinerant, hand-to-mouth existence, turning their hands to whatever work they could find. We have some evidence for singers selling songs at the executions themselves, but they appear to have waited at least until the rope dropped!
For more on street singers of execution ballads see:
Una McIlvenna, ‘Chanteurs de rues, or Street Singers in Early Modern France’, Renaissance Studies 32/4 (2018), Special Issue: Street Singers in Renaissance Europe
Una McIlvenna, ‘When the News Was Sung: Ballads as News Media in Early Modern Europe’, Media History (2016) Special Issue: ‘Managing the News in Early Modern Europe’, eds. Helmer Helmers and Michiel van Groesen: 1-17