Bava Batra, Chapter Six, Mishnah Two
Mishnah two deals with a person who sells something to another person and some of the sold item is found not to be good. In both of these mishnayoth the question asked is can the buyer demand his money back.
1) If a man sold grain to his fellow, the buyer must agree to accept a quarter-kab of refuse with every seah.
a) [If he bought] figs he must agree to accept ten that are eaten by worms for every one hundred.
b) [If he bought] a cellar of wine, he must agree to accept ten jars gone sour in every one hundred.
c) [If he bought] jars in Sharon, he must agree to accept ten which are not fully dry (and therefore are more easily broken) in one hundred.
When a person buys a large amount of a certain item he can expect that most of the items will be pure and in good working order, but he cannot expect that they will all be pure or in working order. If he buys grain and he finds that there is a certain amount of refuse in the grain, as long as the refuse is not more than a quarter-kab (about 350 grams) per seah (8.3 liters), he cant demand his money back. This is about five per cent refuse. Similarly if he buys figs he can expect that some will be rotten; if he buys barrels of wine some will be sour and if he buys jars some will not be made properly. As long as the unacceptable part of the purchase is less than ten per cent, he cannot demand his money back.
[Note: Sharon is on the coast of Israel, between Jaffa and Haifa.]