Shalom,

The Mishnah at the beginning of the Masechta forbids doing business with an idol-worshiper three days before his festival only when the Jew is selling. The Gemara on Daf Vav gives two reasons:

1) When a Jew sells an animal, the buyer might bring it as a Korban to Avodah Zarah. When the Jew causes the idol-worshiper to sin, he himself violates Lifnei Iver, for he is making the other commit an Aveirah.

2) A buyer is happy with his profit and may thank his idol for the good deal. A Jew should not cause that praise.

Both reasons apply especially to selling. When the Jew is buying, the risk is much smaller. Buying goods that last a long time is therefore allowed. Buying perishable items like Yerek is more doubtful because the seller is glad to move them quickly, so some opinions are strict there.

Rashi says that a seller of durable goods is usually not happy about selling, so buying those goods is fine. The Ritva asks why a seller would not be glad, since he now has money for new stock or personal needs. The Ran answers that most sellers of items that last feel some loss when they part with their merchandise, so the Chachamim did not split hairs and allowed buying all lasting goods. Reish Lakish and Rebbi Yochanan argue about whether it is pemitted to give Hana'ah to Avodah Zarah to give indirect benefit to the idol, through the tax that its officials will collect from the shop on the transactions the Jew conducted with them.

As for Tosfos, he sets two limits on Reish Lakish's rule that letting an idol gain is sometimes permitted. In the first answer, Tosfos says that if the benefit is large and public, for example, leaving extra courtyard space in front of an idol, it looks as if the Jew donated land to honor it. Such a clear honor stays forbidden.

The second answer, that of Rabeinu Tam, is that paying the customs fee is allowed only because it shows disrespect for the idol. The payer treats it like any tax office and by reducing the idol to a mere toll thereby belittles rather than honorS it. Where the action gives real honor to the idol, even Reish Lakish would forbid it.

Kol Tuv,

Aharon Steiner