Translation:Shulchan Aruch/Orach Chaim/20
20: Laws of Buying and Selling a Shawl.
1 One who buys a tallit with tzitzit on it from a Jew or from a heathen merchant [and he says that he bought it from a Jew, he's believed] [N"Y laws of tzitzit] it is valid, because since he's a merchant there's a presumption that he bought it from a Jew because he wouldn't do bad to himself. But, if he bought from a heathen that's not a merchant, it is invalid.
2 One does not sell a tallit with tzitzit on it to a heathen because maybe he will 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 heathen is forbidden, unless it's for a short time since then there's no need to worry about this.