I had the same question playing this game for the first time. The portion of the rule book that discusses market stalls should clearly state that only the player who places the market stall in his or her area can use it. It is not expressly stated anywhere. One should not have to read the appendix with details on every helper card to be able to figure this out.You're absolutely right and I'm sorry - it's a mistake of the translation. In the German version it's described precisely, that you can only use your own market stalls. This information unfortunately got lost in the English version ...