1. One who buys a tallit with tzitzit on it from a Jew or from a non-Jewish merchant Rema: and he says that he bought it from a Jew, and he's believed (N"Y laws of tzitzit) it is valid, because since he's a merchant there's a presumption [that he's telling the truth and] that he bought it from a Jew because he wouldn't do bad to himself (and ruin his career by lying to customers). But, if one bought it from a non-Jew that's not a merchant, it is invalid.

2. One can not sell a tallit with tzitzit on it to a non-Jew because maybe he will [wear it, pretending to be a Jew, and] accompany a Jew on the road and kill him. Even to use as collateral and to deposit a tallit with tzitzit on it to a non-Jew is forbidden, unless it's for a short time since then there's nothing to worry about.