You shall not move your neighbour’s boundary… (Deuteronomy 19:14)
While the Torah prohibits infringing another person’s property by moving a boundary marker (hasagat gevul, lit. infringement of boundary), the essence of this law is much more extensive. Rashi (d. 1105) cites another example of hasagat gevul which prohibits planting near a bordering field if water and nutrients will be drawn from the neighbour’s land, reducing its fertility and harming the neighbour’s livelihood (Rashi on Shabbat 85a).
In a similar vein, while the Gemara appears to promote marketplace competition by stating that one is entitled to set up a shop adjacent to an existing shop, it rules that a mill owner may prevent another mill from opening nearby if it will interfere with his business (Bava Basra 21b). Rabbi Yair Chaim Bacharach (d. 1702) held that competition is always permitted provided that taxes are paid to the local authority (Chavot Yair 42).
Rabbi Mordechai ben Hillel (d. 1298) ruled that if there is a shop at the end of a closed alley, it would be forbidden to open a similar shop at the beginning of the alley as it would attract customers in before they reached the other shop (Mordechai, Bava Batra).
Rabbi Yosef ben Meir Migash (Ri Migash d. 1141) limited the restriction on competition to cases where there is no real benefit to the customers. If one shop offers lower prices however, it is permitted to open near a more expensive shop.
In another matter, Rabbi Moshe Isserlis (Rema, d. 1572) ruled in a case where Rabbi Meir of Padua together with Aloizi of Venice published a revised printing of Maimonides’ Mishnah Torah in 1551. Shortly afterwards another Venetian publisher, Antonio Ostinian published his own version undercutting Padua’s. Rema ruled that this was prohibited due to hasagat gevul on the grounds that it would certainly cause Rabbi Padua financial ruin. He ruled that the community should not buy Ostinian’s version until all of Padua’s books had been sold (Rema Responsa 10).
While in general marketplace competition is good, Jewish law also seeks to protect the livelihoods of all. While healthy market competition is good, it must not be at the cost of another’s livelihood.