You're an inmate in Hard Time III, and Schedule I's real products, mixers and effects (read from your own Schedule I) are the prison's contraband economy. Every exchange happens in Hard Time III's own conversations: speech bubbles, with a positive or negative reply.
Deal: lock on and taunt toward someone and pick "Want some of the good stuff?". If they bite, choose the product and a small (1) or large (5) sale. Offer it to a guard who isn't on the take and they'll ask what's in your pocket.
Mix (J): mix product with Schedule I ingredients using Schedule I's own mix map. Effects change, and so does the price.
Computer desks (Library, Hall, Arcade): sit down and press U to log on to YardMail. Order products and mixers from your connect, or accept bulk orders inmates email you. Then find the buyer anywhere in the prison: they'll recognise you and ask for it.
Crew & snitches: after a sale, inmates may ask to join. Turn one down and they might say "If you're not gonna cut me in, I might as well snitch!" and head for the door. Catch them (L) before they leave or you're wanted.
Guards & bribes: a guard right beside you can spot a deal, a hand-over or mixing (never what's just in your pockets). Pay the bribe to walk away clean, or take your chances.
Solo. Needs Hard Time III and Schedule I; nothing from either game is shipped.