Demai, Chapter 2, Mishnah 4. This mishnah deals with the exceptions - with those who are not obligated to separate demai under certain circumstances. The mishnah opens with the law of the nachtomin, the bakers.
The Bakers - A Leniency in Separating:
The nachtomin are retail bakers, and as a rule in the Mishnah a retail seller is obligated to separate demai before selling the produce. With bakers, however, an exception was made: the Sages did not obligate them to separate all of the tithes, but only "kedei terumat ma'aser v'challah" - the equivalent of terumat ma'aser and challah:
The measure of terumat ma'aser - one hundredth of the total. For the tithe itself is a tenth, and terumat ma'aser is a tenth of that.
Challah - a separate obligation that applies to them in any case.
That is all they are required to separate, and they are not obligated to separate ma'aser sheni.
This was permitted to them only when they sell to chaverim and not to the common people, for then they know that the chaver himself will be careful to separate ma'aser sheni. If they are selling to an am ha'aretz, they must separate ma'aser sheni in advance.
The Gemara explains the reason for the leniency: the bakers of that period were under pressure from the government to sell bread at a very low price, and because of the economic pressure they were under, the Sages granted them this leniency.
The Retail Shopkeeper:
Retail sellers, shopkeepers, may not sell demai, and two reasons are given for this:
Since they sell at retail, their profits are greater, and therefore they are expected to be able to bear the obligation of separating demai.
Because children also come to buy from them, and children cannot be expected to separate tithes - therefore the separating falls upon the shopkeeper.
"Mashpi'im b'magasah":
The mishnah lays down a rule: whoever sells in large quantities, that is, whoever sells wholesale, may sell demai. The wholesalers would give a large quantity of merchandise, and it was their practice to give more than the standard measure, and for that reason they did not make a large profit. Therefore the Sages did not place upon them the additional responsibility of separating demai, and it falls instead upon the buyer.
And who are the examples of mashpi'im b'magasah? The wholesalers, the large merchants who sell to the retail shops, and likewise those who sell grain to all the retailers - they too are considered mashpi'im b'magasah.
In summary: in this mishnah we learned the exceptions to the obligation of separating demai: the bakers, whom the Sages obligated only in "kedei terumat ma'aser v'challah" and not in ma'aser sheni - and this because of the economic pressure imposed upon them by the government, and on condition that they sell to chaverim and not to the common people; the retail shopkeeper, who may not sell demai because of his greater profits and because children too buy from him; and in contrast to them the mashpi'im b'magasah, the wholesalers who sell in large quantities and in generous measure, who may sell demai, with the obligation to separate falling upon the buyer.