Demai, Chapter Two, Mishnah Five
This mishnah is a direct continuation of the previous mishnah. At the end of mishnah four we learned that one who sells produce in small measures cannot sell demai, but rather must separate out the tithes before he sells. In contrast, one who sells in large measures can sell demai without separating the tithes, under the assumption that he sells a little extra so that the buyer can separate the tithes himself.
1) Rabbi Meir says: [if produce] which is usually measured out [for sale] in a large [quantity] was measured out in a small [quantity], the small quantity is treated as if it was a large [quantity]. If [produce] which is usually measured out for sale in a small [quantity] was measured out in a large [quantity], the large [quantity] is treated as if it was a small [quantity].
2) What is considered a large quantity? For dry [produce] three kavs, and for liquids, the value of one dinar.
3) Rabbi Yose says: baskets of figs, baskets of grapes, and bushels of vegetableswhen he sells them in lumps, they are exempt [from the rules of demai].
Section one: Rabbi Meir says that something that is normally sold in large quantities, such as grain, is treated as such even in the isolated case in which it was sold in a small quantity. Since it is normally sold in large quantities, one can always sell it demai, even if he is selling a small quantity. The opposite also holds true. If something that is normally sold in small quantities is sold in an isolated case in a large quantity, it is treated as if it was sold in a small quantity and one has to tithe it before selling it. To Rabbi Meir the rule is determined by the nature of that which is being sold, and not by the details of the individual case.
Section two: Three kavs is about three liters. It seems that a large amount is not that actually that largeAlso, a dinar is not a particularly large sum of money. The threshold for being considered a large quantity seems to be quite low.
Section four: When one sells baskets of these types of produce (figs, grapes and vegetables) the exact amount in the basket is not precise. Therefore the seller can sell them while they are still demai, under the assumption that he throws a little extra in so that the buyer can separate the tithes on his own.