Tosefta for Shabbat 121:20

[The produce sold by] a merchant in every place presumed to be Demai, regardless of whether he is a Gentile or a Jew or a Samaritan. What are we talking about here? [This refers to] a situation where they bring him [produce to sell] from a Jew, but if they bring him [produce to sell] from a Gentile or from a Samaritan, it is presumed to be certainly untithed. What is a "merchant"? Anyone who brings [produce] and comes back a second time and a third time (see Shab. 61a:16 re "וְשָׁנָה, וְשִׁלֵּשׁ").