Commentary for Avodah Zarah 11:16

The implication above was that if the idolater has another animal to sacrifice, the Jew who sells him an animal does not transgress the prohibition of placing a stumbling block before the blind. But this contradicts a baraita stated by R. Natan. If one makes forbidden substances available to those to whom they are prohibited, he transgresses, even if the person could have consumed the substance in any case.