We are learning Tractate Demai, chapter 8, mishnah 6. The mishnah deals with a person who buys produce from a siton - a wholesale merchant who collects produce from many different people - and then buys from him a second time.
"Haloke'ach min hasiton vechazar velakach mimenu sheniyah - lo ye'aser mizeh al zeh":
Even though the second purchase was made from that very same wholesaler, one may not take the tithe from one purchase on behalf of the other.
"Afilu me'oto hasug" - even if the produce was taken from the same bin and the same basket, and even though it seems reasonable to assume that it all came from a single source.
"Va'afilu me'oto hamin" - even if it is literally the same species, barley or rye, oats or wheat, one may not assume that the produce came from the same supplier.
And since we cannot assume that it all came from one supplier, we must be concerned that the wholesaler bought part of his merchandise from someone who separates tithes and part of it from someone who does not. The result is that when the buyer comes to separate the tithe of demai from one purchase on behalf of the other, he may be separating from produce that is exempt on behalf of produce that is obligated - from produce that has already been tithed and is exempt - and such a separation accomplishes nothing.
"Ne'eman hasiton lomar mishel echad hem":
The mishnah concludes that if the wholesaler testifies and confirms that all the produce came from one person, he is believed. Once he has provided this information, one may separate demai from one purchase on behalf of the other, and the separation is fully effective.
It emerges that the ruling of the mishnah applies specifically where the wholesaler was not asked, or where the buyer has no access to him at present, so that he is left without the necessary information. But whenever the wholesaler can supply the information and testify that it all came from one person, his word is believed.
In summary: One who buys from a wholesaler twice does not tithe from one purchase on behalf of the other - even from the same bin and even from the same species - out of concern that the wholesaler received his merchandise from different sources, and he would then be separating from the exempt on behalf of the obligated. However, the wholesaler is believed to say "mishel echad hem", they are all from one person, and when he testifies to this, the ruling reverts and one may separate demai from one on behalf of the other.