Tosefta sur Démaï 5:12

[One who] purchases [produce] from a boat [docked] in Jaffa or from a boat [docked] in Caesarea [others have ציפורי = Tzippori or Sepphoris] is liable [in the laws of Demai]. Rabbi Yehudah says, [all produce purchased from] the archway of the Yishuv and the archway of Antipatris and the marketplace of Patros originally was [considered] Demai, because there was a presumption that it was grown in the King's Mountain [cf. Y. Dem. V.8.6]. [But] now, our rabbis have held that [with respect to] all Samaritan towns that are by the side of the "Road" [per Lieberman here, the road running from from Judea to the Galilee], their grains and their legumes are [considered] Demai because they are of Israel [presumably, since Samaritans continue to consider themselves bound by the Written Torah, they may tithe such items, see Dem. 3:4, Dem. 5:9], but the remainder of their produce [as to which tithing is required by the Oral Torah, which Samaritans reject] is certainly untithed. [Note: משתמשין ביהודה, i.e., "used in Judea," should be stricken per the GR"A.]

[With respect to] one who buys vegetables from an outdoor market, behold, this one spends the entire day [deciding what produce to] select, and we need not be concerned; [however] it is forbidden for him to make up his mind [while holding the produce in one's hand but before purchasing it, see Dem. 3:2], [since] he cannot return it, because he needs to tithe, and he cannot tithe it, because [if he changes his mind and returns the produce to the seller] it will be short some quantity [because he has tithed it], and he cannot buy just one item [constituting priestly portion, while returning the rest (per Lieberman)], lest another [buyer] comes and buys [the returned portion], and [unwittingly] tithes already-tithed produce in exchange for untithed produce [see, e.g., Dem. 5:3].