Question: If a bakery sells kneaded dough to customers who is liable to take challah?
Answer: If the baker is Jewish, then the baker is obligated to take challah from the dough (Shulchan Aruch Yoreh De’ah Siman 326 Se’if 2) and there is no further obligation for the customers to take challah from the dough that they buy.
If the baker is non-Jewish, the dough is not obligated in challah in the first place (Shulchan Aruch Yoreh De’ah Siman 330 Se’if 1) because at the time when the dough is made, which is the time when the obligation of challah starts, the dough was in the possession of a non-Jew.
Furthermore, since the dough was exempted at this stage, even if it is now purchased by Jewish customers, it will not become obligated in challah.
In this situation it is the ownership of the dough that will determine whether or not the dough is obligated in challah, and it is inconsequential if the baker/worker who makes the dough is Jewish or non-Jewish (Shulchan Aruch ibid).