This study explored prisoners’ experiences of the illicit economy (IE) and developed a new measure to predict involvement in the IE. Prisoners (n=71) from four UK male prisons were interviewed about their experiences of the IE and completed a new measure of their susceptibility to the IE: ‘The Biddability Scale’ (BIDSCALE). Interviews illustrated that the IE was considered inevitable and pervasive; trading New Psychoactive Substances (NPS) was considered low risk, but caused mental and physical problems; debt was common leading to isolation, transfers or violence. BIDSCALE had 3 reliable factors: assertiveness, compliance and willpower. Higher biddability was associated with risk behaviours such as trading, using NPS, debt and being isolated. Trading in the IE can lead to debt, violence and isolation. Biddability scores may predict risk behaviours. BIDSCALE may be a useful tool to identify and intervene with prisoners most likely to find the IE problematic.