הלוקח טלית מצוייצת מישראל או מתגר עכו"ם [ואומר שלקחן מישראל נאמן] [נ"י הלכות ציצית] כשר דכיון דתגר הוא חזקה שלקחה מישראל דלא מרע נפשיה אבל אם לקח מעכו"ם שאינו תגר פסול':

One who buys a טלית with ציצית on it from a Jew or from a non-Jewish merchant [and he says that he bought it from a Jew, and he's believed] (Nimukei Yosef - Laws of ציצית) 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 harm himself (ie. ruin his career by lying to customers). But, if one bought it from a non-Jew that's not a merchant, it is invalid.

One can not sell a טלית with ציצית 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 טלית with ציצית on it to a non-Jew is forbidden, unless it's temporary since then there's nothing to worry about.