Maasrot, Chapter Five, Mishnah Four
Our mishnah continues to deal with selling produce to a person not trusted to tithe.
1) One may not sell his straw, nor his olive peat, or his grape pulp to one who is not to be trusted in [with respect to] tithes, for him to extract the juice from them.
2) If he did extract the juice he is liable for tithes, but is exempt from terumah, because when one separates terumah he has in mind the fragments which [is] by the sides, and that which is inside the straw.
Section one: Although straw, olive peat and grape pulp are not truly edible and therefore are not liable for tithes, one shouldnt sell them to someone who is not trusted to tithe, if the purchaser is intending to squeeze the juice out of the olive peat or grape pulp, or to search for the left-over grains within the straw. These products would be liable for tithes, and since he doesnt tithe, one cannot sell to him.
Section two: If, nevertheless, he did sell these to someone else, the purchaser must take out the tithes. However, he need not take out terumah because when the original seller takes out terumah for the main-product he will intend to separate terumah on behalf of these by-products as well. For instance, one who takes out terumah from grain, intends to exempt the grain that is found in the broken stalks that werent threshed properly, as well as the grains that are left among the straw. Since the original owner separates tithes for this grain, the purchaser need not do so. The same is true for olives and grapes.
However, the purchaser still needs to separate tithes because the original owner separates tithes based on measurements, and these by-products are not part of that measure.