Before us is Mishnah 5 in the first chapter of Maseches Maaser Sheni. The Mishnah teaches that if one buys any of the following items with Maaser Sheni money, the sanctity of the maaser does not take effect on them:
Water.
Salt.
Produce still attached to the ground.
Produce that cannot reach Jerusalem - that will rot along the way before arriving there.
Since the sanctity of the maaser does not take effect on these items, the money the seller received in exchange for them remains in the sanctity of Maaser Sheni, and he must return it to the buyer who gave him the money.
And if the buyer cannot be located, it falls upon the seller to redeem that money onto something else, and to eat that item in Jerusalem.
Why these items cannot be purchased with Maaser Sheni money:
From the exposition of the verse we learn that one may buy with Maaser Sheni money only items that meet four conditions, all of them derived from that very same verse:
That it be fit to eat.
That it grow from the ground - and an animal too is considered as being nourished from the ground.
That it be able to last until it reaches Jerusalem.
That it be detached from the ground.
One who buys produce with Maaser Sheni money outside of Jerusalem:
If one buys produce with Maaser Sheni money outside of Jerusalem, the sanctity of Maaser Sheni takes effect on the produce, and from now on it must be brought up to Jerusalem. The Mishnah distinguishes between an unintentional act and a deliberate one:
"B'shogeg": if the buyer did not know that he was using Maaser Sheni money, the sale is annulled and the money is returned, since this is a "mekach ta'us" (a mistaken transaction) - at the time of the purchase he did not understand that from now on he would have to bring produce up to Jerusalem instead of money, which is far more burdensome, and therefore the sale is annulled.
"B'meizid": if he knew that he was holding Maaser Sheni money and deliberately bought produce with it - which is forbidden, since one does not buy produce with Maaser Sheni money outside of Jerusalem - the sale is not annulled, and he must bring the produce up and eat it in Jerusalem. He may not even redeem it back onto money.
Ideally, one should not use Maaser Sheni money to buy produce outside of Jerusalem, but only in Jerusalem itself.
And at a time when the Beis HaMikdash does not stand, when Maaser Sheni is not eaten in Jerusalem - "yirakevu" (let them rot): produce upon which the sanctity of Maaser Sheni has taken effect cannot be redeemed back onto money, and therefore there is no choice but to leave it until it rots.
In summary: in this Mishnah we learned that water, salt, items attached to the ground, and produce that cannot reach Jerusalem are not acquired with Maaser Sheni money, and therefore the money in the seller's hands retains its sanctity and is returned to the buyer (and if he cannot be located, it is redeemed onto something else that is eaten in Jerusalem); we examined the four conditions derived from the verse - fit to eat, growing from the ground, able to last until Jerusalem, and detached; and the distinction between an unintentional and a deliberate purchase of produce outside of Jerusalem, as well as the law at a time when the Beis HaMikdash does not stand.