Yoma 9A

לפי ששלח בכל גבולי ישראל, וראה שאין מפרישין אלא תרומה גדולה בלבד.

This is because Yoḥanan the High Priest sent emissaries throughout all the areas located within the borders of Eretz Yisrael to assess the situation and saw that the people were separating only teruma gedola and were neglecting to separate tithes. Therefore, he issued a decree that anyone who purchases produce from an am ha’aretz must be concerned about the possibility that it was not tithed and is required to tithe it. Since even an am ha’aretz separates teruma gedola, the bakers who purchased grain from them were not required to do so.

מעשר ראשון ומעשר עני נמי לא, המוציא מחבירו עליו הראיה. אלא מעשר שני, נפרשו ונסקו וניכלוה בירושלים?

And granted, bakers need not separate first tithe and poor man’s tithe due to the principle: The burden of proof rests upon the claimant. Neither first tithe, given to Levites, nor poor man’s tithe, given to the poor, is sacred. It is merely the property of the Levite and the pauper, respectively. Since with regard to doubtfully tithed produce, by definition, there is no certainty that one is actually required to tithe it, if the Levite or the pauper should seek to take possession of the gifts, they must first prove that in fact the produce was not tithed. However, with regard to second tithe, why are the bakers exempt? Let them separate second-tithe from the produce, take it up to Jerusalem, and eat it in Jerusalem, which is the halakha with regard to anyone else who purchases doubtfully tithed produce.

אמר עולא: מתוך שפרהדרין הללו חובטין אותן כל שנים עשר חדש, ואומרים להן: מכרו בזול! מכרו בזול! לא אטרחונהו רבנן. מאי פרהדרין? פורסי.

Ulla said: It is because these parhedrin, government appointees, beat the bakers throughout the entire 12 months of their tenure and tell them: Sell your baked goods cheaply, sell them cheaply. Since the officers insist that the bakers refrain from raising their prices, the rabbis did not further burden them with the exertion of separating second tithe from a large quantity of grain and taking it to Jerusalem, as they would be unable to raise their prices to cover the cost of the lost grain and the trip to Jerusalem. Since the presumptive status of the grain is that it was tithed, and the obligation to tithe doubtfully tithed produce is a stringency, the rabbis exempted the baker from the obligation to do so. What is the meaning of parhedrin? These are royal appointees [pursei] charged with performance of different tasks.