Maaser Sheni, Chapter 3, Mishnah 13. There are many ways to explain this Mishnah, and we will follow the approach presented here. The Mishnah discusses someone who sells wine in Jerusalem, where the buyer - who is purchasing the wine with Maaser Sheni money - wants to arrange the sale in such a way that the jugs themselves will not be considered sanctified with the sanctity of Maaser Sheni and will not require redemption.
How can this be accomplished?
"Mefatei'ach u'me'areh la'gas" - according to the first opinion in the Mishnah, the seller must open the jugs and pour the wine into the pit of the winepress, so that it is clearly demonstrated that he is not selling him the jugs but only the wine.
And Beis Hillel say: "Mefatei'ach v'eino tzarich le'aros" - it is enough to open the jugs in order to make clear that the seller is not selling him the jugs, and there is no need to actually pour out the wine. The very fact that the jug is open shows that the jugs are not being sold with Maaser Sheni money.
"Bameh devarim amurim":
When does Beis Hillel say that opening the jug is enough to prevent it from becoming sanctified with the sanctity of Maaser Sheni?
"B'makom she'darko limkor sesumos" - in a place where the custom is to sell wine in sealed jugs. There, by opening the jug the seller is doing something out of the ordinary, and in that way he gives a sign and makes it clear that he is not selling the jug.
"Aval b'makom she'darko limkor pesuchos - lo yatza kankan l'chulin" - in a place where the custom is to sell the jugs open, the opening is no sign at all, since nothing has been done beyond the ordinary practice. The jug is included in the sale and does not leave it. Under these circumstances the jug is not secondary to the wine, and since the buyer pays for it with Maaser Sheni money, it has a sanctity of its own: at the moment they declare that the wine in the jug will be Maaser Sheni, the sanctity of Maaser Sheni takes effect on the jug as well.
Two additional ways to leave the jug as unsanctified produce:
Selling by measure: If one wishes to make things harder on himself and sell the wine by measure, that is, to say "such and such a quantity of wine I am selling for such and such a price" - whether the jugs are sealed or not sealed, the jug remains unsanctified and is not included in the sale.
Rabbi Shimon says: "Af ha'omer la'chaveiro: chavis zo ani mocher lach, chutz mi'kankanim" - one who says explicitly that he is selling the jug of wine while excluding the jugs themselves from the sale, there too the jug remains unsanctified. It is not included in the sale and is not acquired with Maaser Sheni money, and therefore it does not become sanctified with the sanctity of Maaser Sheni, and the maaser takes effect on the wine alone.
In summary: This Mishnah teaches how one can buy wine in Jerusalem with Maaser Sheni money without the jug becoming sanctified with the sanctity of Maaser Sheni: according to the first opinion - by opening the jugs and pouring the wine into the winepress; and according to Beis Hillel - opening them is enough. And even that applies specifically in a place where the custom is to sell sealed jugs, for there the opening is a recognizable sign; but in a place where the custom is to sell them open, there is no sign in this, and the jug is included in the sale and becomes sanctified with the sanctity of Maaser Sheni. And we learned two additional ways: selling the wine by measure, and according to Rabbi Shimon - explicitly excluding the jug from the sale.