Q: In certain industries, the mark-up can reach 300 percent. Is there any profit limit in halachah?
A: It is prohibited to acquire and hoard large quantities of basic produce in Eretz Yisrael or in a place that is predominantly Jewish, since this significantly reduces the supply and inflates the price. However, a person is allowed to withhold and hoard what he grows and sell it at high market time. In a time of famine, a person may not hoard more than his household needs for a year, even of his own produce (C.M. 231:24-25; Pischei Choshen, Geneivah 14:13).
Similarly, beis din can punish and fine those who inflate prices beyond the normal market range, such as a cartel (C.M. 231:21).
A person who hoards or inflates prices in a prohibited manner is tantamount to one who lends with interest, since he violates “vechai achicha imach” (Sma 231:43).
If fish sellers raise their prices significantly, beis din can decree that the community not buy fish for Shabbos for a number of weeks until the price returns to normal (Mishnah Berurah 242:2).