Yaakov Yosef A

A: Gemara Arachin 29a, Rambam Maaser Sheini 2:2, Yoreh Deiah siman 331 se’if 133.

B: The mitzvah of separating maaser sheini does in fact apply nowadays, but not the mitzvah of eating it. This is derived in the Gemara from a possuk that compares maaser sheini to korbanos. See Makkos 19a.

C: The reason we do it this way is to prevent accumulation of maaser sheini that is forbidden to be eaten – “mishum takalah”. The reason it works even with a ‘prutah’ is because m’doraisa the prohibition on cheating in transactions doesn’t apply to Hekdesh (including maaser sheini). לא תונו איש את עמיתו meaning ‘colleague’, excludes ממון גבוה. The Rabbanan established a prohibition on cheating Hekdesh, but limited it to when the Bais Hamikdash existed so as to avoid accidentally using Hekdesh nowadays which is forbidden.

Hope this clarified things for you.