Demai, Chapter Two, Mishnah Four
Our mishnah deals with how various food professionals observe the laws of demai.
1) Bakersthe sages did not obligate them to separate [from demai produce] any more than suffices for terumat maaser and for hallah.
2) Grocers may not sell demai [produce].
3) All [merchants] who supply in large quantities may sell demai.
a) Who are those who supply in large quantities? Those such as wholesalers and grain-sellers.
Section one: The rabbis were lenient in mandating bakers to separate all of the tithes from demai (produce they bought from an am haaretz) because bakers barely make a profit. If the sages had been strict, bakers might have been left with a choice: go out of business or ignore rabbinic law. The baker did have to remove terumat maaser (the terumah taken from the tithe) and the hallah. These are of a higher level of holiness, but are not necessarily a large amount of produce. However, the baker did not have to take out second tithe.
Section two: Grocers, on the other hand, cannot sell demai until all the tithes, meaning terumat maaser, hallah and second tithe, have been separated. Evidently grocers made a greater profit than did bakers and hence the rabbis were more stringent with them.
Section three: Anybody who sells in large quantities is allowed to sell demai, under the assumption that he is providing a little extra so that the buyer can separate the tithes himself. The mishnah enumerates two types of merchants who usually sell in large quantities: wholesalers and grain merchants. In contrast, a grocer, referred to in section two, sells in more precise measurements and hence has to separate the tithes himself.