Gemara

6a (Beraisa - R. Noson): "V'Lifnei Iver Lo Siten Michshol" forbids extending wine to a Nazir or a limb from a living animal to a Nochri.

13b (Mishnah): One may not sell the following to Nochrim frankincense, white chickens...

All other things are permitted Stam, but forbidden if specified.

14a (R. Yitzchak): The Mishnah discusses clear frankincense (a specific kind of frankincense).

(Beraisa): Regarding all of these, one may sell a whole bundle.

(R. Yehudah ben Beseira): A bundle weighs at least three Manos.

Question: We should be concerned lest the Nochri sell to others, and they will offer it to idolatry!

Answer (Abaye): We are commanded "Lifnei Iver...", but not 'Lifnei d'Lifnei' (causing someone to cause someone else to sin).

Nedarim 62b: Rav Ashi sold a forest to priests who served fire.

Question (Ravina): You transgress "v'Lifnei Iver"!

Answer (Rav Ashi): Most wood is burned (not for idolatry).

Pesachim 40b: A ship carrying wheat capsized in a river (before Pesach). Rava permitted to sell the wheat to Nochrim.

Question (Rabah bar Livai - Beraisa): If Sha'atnez was lost in a garment, one may not sell it to a Nochri (lest the Nochri resell it to a Yisrael)!

Rava retracted and commanded to sell it to Yisraelim, a bit to each buyer, in order that it would be finished before Pesach.

Rishonim

Rosh (1:2): The Beraisa discusses giving Ever Min ha'Chai to a Nochri to teach that Lifnei Iver applies to even to Nochrim.

Rosh (14): Abaye taught that we are not commanded 'Lifnei Lifnei', i.e. regarding Nochrim. Regarding Yisrael we are concerned for this, like we say in Pesachim, that one may not sell to Nochrim if they will sell to Yisrael (and cause Yisrael to transgress).

Rosh (15): One may not sell to a Nochri a book of his prayers (some texts - idolatry), due to Lifnei Iver. Rav Ashi sold a forest to priests of idolatry. It was not Lifnei Iver because most wood is burned not for idolatry. If not for this, it would be forbidden. Also Re'em explained like this.

Tosfos (14b DH Chatzav): R. Baruch says that since we forbid everything else when specified, one may never sell frankincense to priests, for it is as if they specified. Surely they want it for idolatry! One who sells it transgresses Lifnei Iver. One may not sell wax to any Nochri on his festival. Surely it is permitted on other days. If a Yisrael bought Kelim of idolatry after a Nochri blemished and was Mevatel them, he may not sell them to a Nochri like they are, until he melts them down. A blemish would not stop a Nochri from putting wine in it to praise idolatry. One may not sell to priests Pasul Seforim that can be used for idolatrous rites due to Lifnei Iver. One may not sell them to any Nochri, for he will surely give or sell them to a priest.

Poskim

Shulchan Aruch (YD 151:1): One may not sell things special for a kind of idolatry to its worshippers.

Taz (1): If it is a Safek if it is needed for idolatry, we are lenient.

Shach (2): it is forbidden even not on the day of his festival, even nowadays, even to a Nochri who is not a priest.

Shulchan Aruch (ibid): If he buys so much that it is evident that he buys for business, it is permitted.

Taz (3 and Shach 3): We are not concerned lest he resell it to others, for this is Lifnei d'Lifnei. If there is concern lest a Yisrael transgress, we are concerned even for this, like we find in Pesachim.

Gra (2): Why did the Rambam omit this Heter?

Shulchan Aruch (ibid): Similarly, if he says that he needs it for other matters, and the Yisrael knows that this is true, it is permitted. One may sell things special for idolatry together with other things, e.g. clear frankincense amidst black frankincense, and we are not concerned lest he pick out the clear frankincense amidst the black.

Rema: This refers to a priest or idolater established to burn frankincense to idolatry. One may sell (even clear frankincense) to a Stam Nochri.

Beis Yosef (DH Kasvu b'Sefer) Sefer ha'Terumos forbids selling Pasul Seforim even to a Nochri who is not a priest, for he will surely sell it or give it to a priest. This is like wood, which one may not sell to priests to burn for their sacrifices.

Note: This is like Tosfos 14b. I do not understand this, for we permit Lifnei d'Lifnei regarding Nochrim! In Nedarim we said that selling wood to priests would be Lifnei Iver if not that most is burned not for idolatry.

Bedek ha'Bayis: Terumas ha'Deshen (2:112) forbids selling to priests parchment and ink, and he was unsure about quills (for they do not end up in the Sefer).

Rema: One may not sell water to a Nochri if he knows that he wants it to immerse.

Beis Yosef (DH Kasav Od): R. Yerucham says that if priests request water to immerse or to sprinkle on people, one may not give to them. If they request Stam, one may give to them.

Beis Yosef (DH uv'Kisvei): The Terumas ha'Deshen (27) says that one may not sell to priests books of their songs to idolatry, but if one does not know what is in the Sefer, it is permitted, for most of their Seforim are civil laws, medicine, astronomy, geometry, and music. Perhaps one may sell even a religious book to a Stam Nochri, for the Nochri is not commanded about Lifnei Iver (he will sell it to a priest). Orchos Chayim says that if clothes were made for regular people, and the priests want to use them for idolatry, some are stringent and some permit. I apply the Mishnah 'other things are permitted Stam, but forbidden if they specify.' Some are stringent because there are garments that are known to be bought for covering the altar. Garments that are just for the priests are permitted (Stam).

Gra (6): Tosfos derives that the Reisha of the Mishnah discusses a priest.

Gra (7): R. Yerucham says that a Nochri established to offer to idolatry is like a priest.

Pischei Teshuvah (1): Chavos Ya'ir (136) says that since Nochrim are commanded about monetary laws, a Yisrael who bribes a Nochri judge transgresses Lifnei Iver. Based on this, one may give to the judge's slave and tell him to give it to the judge (unless this would cause a perverted judgment and the giver will receive more than he deserves), for this is Lifnei d'Lifnei. It is not a probem of Shlichus, for a Nochri cannot be a Shali'ach.

Rema (EH 5:14): One may sell animals and roosters to a Nochri, even though the Nochri will castrate them. Some forbid. All agree that one may give an animal to a Nochri if he will not castrate it himself, rather, he will give it to another Nochri.

Beis Shmuel (19 and Gra 39): This is Lifnei Lifnei, which is permitted. Even if he made him a Shali'ach, Ein Shali'ach l'Devar Aveirah.

Yad Efrayim, citing Teshuvas Pnei Yehoshua (3): Nowadays Lifnei Iver regarding Nochrim is mid'Rabanan, because Hash-m permitted their Mitzvos (Rashi 12). Therefore, one may follow the lenient opinion in the Rema when needed.

Note: Perhaps he learns from Rashi 6a DH v'Lifnei. I find this very difficult. It seems that the conclusion (3a) was that Hash-m did not permit their Mitzvos, rather, He merely diminished their reward for them!