(a) According to Beis Hillel, if the owner wants to Ma'aser all the barrels from one barrel after having sealed them, it will suffice to simply open them - Beis Shamai require him to pour them back into the wine press before declaring part of it Ma'aser Sheni (see Tiferes Yisrael).
(b) The Mishnah qualifies the earlier ruling rendering the barrel Kodesh if the owner declared the wine Ma'aser Sheni after sealing the barrel - only if it is customary to sell the barrels sealed, but not where the wine is normally sold from open barrels ...
(c) ... as we learned in the first Perek regarding the purchase of wine with Ma'aser Sheni money in Yerushalayim.
(d) The barrel remain will remain Chulin, however, even where it customary to sell the barrels sealed - if the seller makes a point of selling the wine per measure, rather than by the barrel (see Tiferes Yisrael).
(a) Rebbi Shimon states that in a case where Reuven says to Shimon that he is selling the barrel of wine apart from its barrel - the barrel remains Chulin.
(b) In actual fact, he is referring to the case in the previous Mishnah ('[Im] mi'she'Gafan [Kara Shem Ma'aser] Kanah Ma'aser') - where he previously declared the contents of the barrel Ma'aser except for one Revi'is, and it is that Revi'is (of Tevel [see Tiferes Yisrael]) that he is now selling Shimon, stipulating that he is retaining the (sealed) barrel for his Ma'aser Sheni, then the barrel remains Chulin (see also Tiferes Yisrael).