Sukkah 30B

וקרקע אינה נגזלת. הלכך לגזזוה אינהו, כי היכי דליהוו יאוש בעלים בידייהו דידהו ושינוי הרשות בידייכו.

and land is not stolen. When one seizes land, the land remains the property of its original owner, even if that owner has despaired. In this case, there is concern that these myrtle branches were stolen from Jews. Therefore, let the non-Jews cut the myrtle branches, so that the despair of the owners will be when the myrtle branches are still in the hands of the non-Jews and the change of possession will be accomplished through their purchase and transfer into your hands. The combination of owner’s despair and change of possession will render the myrtle branches the property of the merchants, and it will not be a mitzva fulfilled by means of a transgression.

סוף סוף כי גזזו אוונכרי, ליהוי יאוש בעלים בידייהו ושינוי הרשות בידן! לא צריכא, בהושענא דאוונכרי גופייהו.

The Talmud asks: Ultimately, even when the merchants cut the myrtle branches, let it be a case of despair in their hands, and the change of possession is accomplished through the purchase and transfer of the myrtle branches into the hands of the buyers. Why did Rav Huna advise them to have the non-Jews cut the myrtle branches? The same result is achieved through their sale. The Talmud answers: No, it is necessary for Rav Huna to advise the merchants to allow the non-Jews to cut the myrtle branches only with regard to the myrtle branches of the merchants themselves, which will not undergo another change of possession. The only way to ensure that the merchants are fulfilling the mitzva with myrtle branches that belong to them is to have the non-Jews cut them and have the change of possession accomplished through the purchase from the non-Jews.