Previously-suggested extortion (“The practice of extorting money or other property by the use of force or threats”) is quite appropriate; also consider verb shake down (“To get money from someone using threats”).
Edit: As noted in a comment, as a noun shakedown means “an act or instance of shaking someone down; especially : extortion”. Some other terms related to a policeman forcibly taking money from a salesman might include:
• graft, “Illicit profit by corrupt means, especially in public life”, “A cut of the take”, “A bribe, especially on an ongoing basis”
• squeeze (mentioned earlier by Daniël), “To oppress with hardships, burdens, or taxes; to harass”
• put the arm on, “Fig. to apply pressure to someone. [eg] John has been putting the arm on Bill to get him to cooperate.”
• muscling in, “[using] power or influence, esp. when based on force or threats of force”
• on the take, “trying to profit in a personal and usually financial way from a situation”, which might apply if the policeman knows of infractions but is turning a blind eye for a split of the stand's profits.
Note, cambridge.org has thesaurus pages for specific topics, including Cheating & tricking which may be relevant.