Tosefta for Demai 4:8

[One who] purchases [produce] from a caravan [see Jastrow, cf. Dem. 4:7 (חַמָּרִים)] in Tyre or from the stores [see Jastrow (מוגירות)] in Zidon is exempt [from the laws of Demai], [but one who purchases] from the stores in Tyre or from a caravan in Zidon is liable. Rabbi Yosei son of Rabbi Yehudah says, [one who] buys from the stores in Tyre -- and there is no need to say "from a caravan" but [even from] a single donkey-driver in Tyre -- is liable. Rabbi Yehudah says, [produce purchased from] a caravan coming down to Cheziv is liable because the presumption is that it is (i.e., originated) from the Galilee [and thus is carrying produce grown in Israel, which must be tithed]. And the sages say, behold, it is presumed exempt until it is known [with certainty] from where it came.