Siman 20
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א הַלּוֹקֵחַ טַלִּית מְצֻיֶּצֶת מִיִּשְׂרָאֵל אוֹ מִתַּגָּר עַכּוּ"ם וְאוֹמֵר שֶׁלְּקָחָן מִיִּשְׂרָאֵל נֶאֱמָן (נ"י הִלְכוֹת צִיצִית) כָּשֵׁר, דְּכֵיוָן דְּתַגָּר הוּא חֲזָקָה שֶׁלְּקָחָהּ מִיִּשְׂרָאֵל דְּלֹא מֵרַע נַפְשֵׁיהּ אֲבָל אִם לָקַח מֵעַכּוּ"ם שֶׁאֵינוֹ תַּגָּר, פְּסוּלָה.
1 Laws of Buying and Selling a טלית. Containing 2 Se'ifim: 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.
ב אֵין מוֹכְרִין טַלִּית מְצֻיֶּצֶת לְעַכּוּ"ם שֶׁמָּא יִתְלַוֶּה עִם יִשְׂרָאֵל בַּדֶּרֶךְ וְיַהַרְגֶנּוּ, אֲפִלּוּ לְמַשְׁכֵּן וּלְהַפְקִיד טַלִּית מְצֻיֶּצֶת לְעַכּוּ"ם אָסוּר, אֶלָּא אִם כֵּן הוּא לְפִי שָׁעָה דְּלֵיכָּא לְמֵיחַשׁ לְהָא:
2 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.