Tosefta על פסחים 99:15

One who purchases from a merchant (=תגר, cf. Pes. 50a:15), and he says, this one is "fixed" (i.e., tithes have been taken) and this one is not fixed, even as to the one [that he says is] fixed, we don't believe [him]. [However,] one who typically sells Demai is believed.