Responsa for Menachot 85:8

ת"ר הלוקח טלית מצוייצת מן השוק מישראל הרי היא בחזקתה מן העובד כוכבים מן התגר כשרה מן ההדיוט פסולה

if he bought it from a gentile, who was a merchant, it is valid,<span class="x" onmousemove="('comment',' For the merchant would not risk his reputation as an honest dealer by passing off the imitation blue for the genuine.');">⁹</span> but if he was a private individual it is invalid.<span class="x" onmousemove="('comment',' For the gentile may have dyed the thread himself, in which case it obviously could not have been dyed for the purpose of the precept.');">¹⁰</span>

Teshuvot Maharam