If true, how old was the practice? Was there a time where their powers were at a peak? How powerful was this peak? Any other explanations of guilds in China are welcome.
There is a pretty extensive article here , the author calls them hanghui and apparently they started appearing around the 16th century. They were also often viewed as religous fraternities as well as economic organisations.
Interestingly, "hui" is also used to refer to secret societies and clans, perhaps someone else knows how these terms are related and whether definitions are just fuzzy.
Would I be wrong in assuming that the Manchu 8 Banners system eventually came to resemble trade guilds in the 19th century? (phrased it as a question since I don't have sources)