My Chabura is arguing whether there would be an Issur of Mi Shepara to back out of buying cigarettes (or the like) after the buyer paid. On the one hand smoking is Assur. On the other hand the only cases we’ve found that don’t get a Mi Shepara are cases where the purchase itself is an Issur, while here the purchase itself isn’t an Issur, it’s what you do with it afterwards. Or maybe we say only the Mocher has an Issur since he’s Over on Mesayaya, while the buyer can just choose to not smoke them. He can even throw them out immediately if he doesn’t want to be Over on Al Tishkon Besoch Beisecha Avlah.
In this case the prohibition of Me Shepara will apply, and after the money is given in payment, it is not permitted for either side to back out.
Although it is not permitted to smoke and damage one’s health, this does not mean that the act of selling or buying cigarettes is a prohibition.
The Kesav Sofer (Yoreh De’ah 83) explains that the prohibition of facilitating a sin (mesayeia) applies only during the act, and not prior to an act of sin, such as selling an animal to an idolater – if he already owns suitable animals so that the prohibition of Lifnei Iver is not applicable. It is only forbidden to give assistance during the actual performance of the forbidden act (see Binyan Tzion 15).
Thus, the Binyan Tziyon (15; see Machatzis Ha-Shekel 163) explains that the prohibition of giving food to somebody who will not recite a berachah is not clear-cut, because of the possibility that the prohibition of mesayeia applies only when the forbidden activity is actually taking place. Assistance given beforehand might not fall under the prohibition, which is why the Magen Avraham (163:2) was unsure whether giving food to one who will not recite a berachah is included in the prohibition (but see Yad Malachi 361 and Bi’ur Ha-Gra, Yoreh De’ah 151:8, who apply the prohibition of mesayeia even to granting assistance before the transgression).
Based on the same distinction, the Netziv (Meishiv Davar, Vol. 2, 31-32) permits a Jewish butcher to sell non-kosher food to non-observant Jews. Given that they can purchase meat elsewhere, lifnei iver does not apply, and since the sale takes place before the transgression even the prohibition of mesayeia will not apply. The Netziv likewise permits a rabbi to officiate at a wedding of a couple who will probably not observe the family purity laws; the couple will violate these laws even if the rabbi does not perform the wedding (so lifnei iver does not apply), and mesayeia does not apply since the rabbi’s involvement occurs before the violation.
The same will apply to selling cigarettes, which can be obtained elsewhere (no Lifnei Iver), and which does not involve an actual transgression.
If there would be a full prohibition in making the sale, no Mi Shapara would apply, because retracting from the sale would not be out of choice but rather out of obligation. This will be true for instance of a sale on Shabbos. However, as explained above, in the case of selling cigarettes there is no full prohibition, and therefore the principle of Mi Shepara will apply.