It's often mentioned in the books that the slavers are a sub caste of the merchants. And despite Norman being very anal with such details, it is never mentioned that castes like bakers, cloth makers, etc, are themselves a sub-caste of the merchants.
Selling and trading is the principal skill and activity of merchants. It's not the case for a baker whose skill is to make bread. Also, a member of the baker caste might be only a employe of a well established banker and and never have a contact with customers. A peasant will sell his production. But who would consider him as being of the caste of the merchants and even, as a sub caste of the merchants? But the peasant might sells his produces to a merchant who will resell, trade, or speculate on it for a profit. Same for a wine maker. He can sell his wine to a merchant who might specialize in such trading and will be labelled as a "wine merchant". A metalworker sells items he made as well with his services. But you never see him mentioned as part of the Merchant Caste in the books.
A scribe and a physician can also rent a stall or a store owned and administrated by the merchants. Still they cannot be counted as merchants themselves. The merchant caste administrates many port cities, its high council raise taxes. But reporting to the Merchants doesn't mean you belong to their caste. Maybe the high council of the Merchants could attempt to administrate the sales made by the bakers, but if the Council of the Bakers agreed to this form of control (could lead to some good conflict RP in a SL Gor city), still, why would its members become official members of the caste...? Also, it would sound logical, for example, that in a city administrated by the Merchant Caste, a tax get perceived on all sale transactions. But paying taxes wouldn't make you a Merchant neither.
We have this quote in Nomads:
"It might be mentioned, for those unaware of the fact, that the Caste of Merchants is not considered one of the traditional five High Castes of Gor the Initiates, Scribes, Physicians, Builders and Warriors. Most commonly, and doubtless unfortunately, it is only members of the five high castes who occupy positions on the High Councils of the cities. Nonetheless, as might be expected, the gold of merchants, in most cities, exercises its not imponderable influence, not always in so vulgar a form as bribery and gratuities, but more often in the delicate matters of extending or refusing to extend credit in connection with the projects, desires or needs of the High Councils. There is a saying on Gor, "Gold has no caste." It is a saying of which the merchants are fond. Indeed, secretly among themselves, I have heard, they regard themselves as the highest caste on Gor, though they would not say so for fear of rousing the indignation of other castes. There would be something, of course, to be said for such a claim, for the merchants are often indeed in their way, brave, shrewd, skilled men, making long journeys, venturing their goods, risking caravans, negotiating commercial agreements, among themselves developing and enforcing a body of Merchant Law, the only common legal arrangements existing among the Gorean cities. Merchants also, in effect, arrange and administer the four great fairs that take place each year near the Sardar Mountains."
I went through the quotes shared by Fogaban on his website, The Gorean Cave and it appears in these quotes that merchants specialize in buying/selling/trading goods,also negociate commercial agreements, administrate fairs and cities, open trade roads, have financial activities. But they never produce something themselves, with the exception of the slavers who buy slaves, train and sell them, sometimes, breed them. There is even some mention of merchants trading slaves without considering themselves as slavers. The difference would be, I think, that the slavers do train slaves.
In fact, it seems that in the books, all these castes, such as potter, bakers, etc, are globally referred as artisans, but still, never as being a sub caste of the merchants. I tend to think that Norman, in one of the 33 books, with all the details he gave about the merchant caste, would have mentioned this point.
It's why, it sounds to me that considering different artisans as merchants is more an Earth consideration where we tend to include everyone having a commercial activity as a store keeper/sale man. We ignore, doing this, the importance given to the caste which is more than a job. In SL-Gor, we tend to consider the caste like a matter of job which explains why people tend to change caste like they would change of job in their real life. But in the books, the caste is merely a matter of birth. The caste is a part of your identity in Gor, even you -seldom- take a companion which is not of your caste, even, and it's unlikely that a physician will FC a woman of the baker caste, nor will he give his daughter to a butcher for a FCship contract.
I would love seeing more real merchant activity other than store keeper in the SL-Gor cities, with merchants trading all sort of goods or just one, trying to gain in importance and influence amongst the Caste, their council extending its influence over the high council of the city...