Igrot Moshe Choshen Mishpat 1:72

It is not necessary that the custom be based on the sages, nor even based on Jews, for even if gentiles established the custom – as, for instance, where they are in the majority in that location – it is still Torah law that the standard should follow the custom, for absent another provision it is as if they conditioned [the contract] on the prevailing custom… therefore it does not matter who it is who has established the custom. [translation by Hekhsher Tzedek "Al Pi Din" by Rabbi Avram Israel Reisner for Heksher Tzedek Commission of RA/USCJ]