Tractate Bechoros, Chapter 4, Mishnah 9 - The last of the three specific suspects in our chapter: people suspected of various transgressions for financial gain, and how the Sages penalized them so that they would not profit and could not be relied upon in the future.
The one suspected of selling Terumah:
Here we have a person suspected of selling Terumah produce - crops that were supposed to go to a kohen and be eaten only by kohanim - and he sells them in the market as if they were ordinary chullin, without any restrictions. Even if it is a kohen who received the Terumah, it is still forbidden to be eaten by a non-kohen. Terumah is much cheaper than chullin, since it is received for free and is subject to restrictions; hence a situation may arise where a person sells Terumah claiming it is ordinary food.
A person who was caught doing this in the past, and now wants to sell other food items - how do we deal with him? The Tanna'im disagree on this:
Rabbi Yehuda: We do not buy any food from him at all, "afilu mayim umelach" - even water and salt. Now, water and salt have no connection to Terumah: there is no such thing as non-kosher water or non-kosher salt, nor do they need to be tithed. Nevertheless, the penalty is sweeping - we do not buy any food items from him whatsoever.
Rabbi Shimon: Qualifies this. While it is true that we should not buy food from someone suspected of selling Terumah as chullin, this only applies to food that has the potential of being forbidden due to Terumah - such as a product containing a component made of oil, however small it may be. However, something that has no connection to Terumah at all, such as salt or water, may certainly be bought. It is like forbidding the purchase of a used car from him: both salt and a used car have absolutely no connection to Terumah.
"Zikas terumah uma'asros" - a connection to Terumah and tithes - Rabbi Shimon's addition:
The halachah follows Rabbi Shimon. It is worth noting that Rabbi Shimon mentions both Terumos and tithes, thus adding another element. "Ma'asros" - tithes, here refers to Ma'aser Sheni, which is separated in the first, second, fourth and fifth years of the seven-year Shemittah cycle. The law of this tithe is that it must be brought up to Jerusalem and eaten there, or redeemed onto coins and the money brought to Jerusalem.
Here, then, we have a farmer who says to himself: "This year my crop yielded a million dollars. I do not intend to bring a hundred thousand dollars worth of produce up to Jerusalem to eat it there - that is unnecessary. I will simply sell it as ordinary chullin right here in the market" (the numbers are rounded for simplicity). This person is caught in his corrupt act, as he seeks to make a financial profit.
From now on and forever we do not buy food from him, but the halachah follows Rabbi Shimon: the prohibition applies to food items that have some connection to Terumos and tithes.
The word the Mishnah uses is "zikah" - connection, and it is a borrowed term. Originally, the term refers to the halachic bond tying a widow who awaits yibbum or chalitzah - she is tied to her brothers-in-law until this bond is dissolved. Here it means any connection to Terumos or tithes. And when there is no connection at all, such as with salt and water, there is no restriction on buying foods and groceries from this person, even though he was previously caught selling Terumos and tithes illegally.
In summary: In this Mishnah we learned about one suspected of selling Terumah as chullin. According to Rabbi Yehuda, the Sages penalized him with a sweeping penalty - we do not buy any food from him, even water and salt. Rabbi Shimon qualifies this and rules that the prohibition only applies to items that have a connection to Terumos and tithes, and this is how the halachah is ruled. We also discussed the meaning of "ma'asros" - tithes in his words - referring to Ma'aser Sheni, and the origin of the term "zikah" - connection.