Monster parts are listed as a precious material on the tables of trade goods; Given that only 15% of merchants specialise in precious materials and only 10% of those deal in monster parts, the chance of a given merchant specialising in monster parts is only 1.5%. But worse, there's a footnote that says you should roll on the area's wandering monster table to find out which kind of monster a merchant deals in - which means that the odds of a merchant buying or selling the specific special component you're interested in is tiny at best, and zero if you're in an area where the monster is not endemic.
1. You are correct that only 1.5% of merchants will default to being monster parts sellers. However, a merchant can be persuaded to transact as a seller of precious merchandise with a reaction roll of 12+. That means at least 1 in 36 merchants (about 3%) can be persuaded. Some minor modifiers to the reaction roll can raise this to 25% fairly easily. [The merchant is assumed to be making arrangements through his network of traders to provide the goods, etc.]
2. The intent of the rule is that each merchant who deals in the monster parts deals in monster parts generally. The roll on the wandering monsters table is to determine what he has on hand to sell or is interested in buying at that moment, without requiring to be persuaded.
3. It is an open question, not answered in the rules as written, whether a merchant who is persuaded to sell monster parts, is persuaded to sell random monster parts or specific monster parts. E.g. if you persuade a merchant to sell "hides, furs" that's all that matters, but if you persuade a merchant to sell "monster parts" are you getting whatever he can find, or what you specifically need? How ought we resolve this ambiguity?
- Option A: If you persuade a merchant to sell monster parts, you persuade him to sell exactly what you needed. This seems somewhat hard to swallow from the point of view of verisimilitude.
- Option B: If you persuade a merchant to sell monster parts, you roll randomly to sell what he is able to get his hands on to sell you. That is probably more realistic.
- Option C: A reaction roll of 12+ is required for any monster parts, 15+ for specific monster parts.
> In such a climate, I'm amazed there's a trade in special components at all... Which makes me think I'm misunderstanding something. I had always assumed that mages have to hire adventurers to fetch the special components they need... But could they equally just walk into a town and buy what they need on the open market? Why, then, does the practice of hiring adventurers to obtain special components exist at all?
Let's separate this into two questions. The first is "are monsters hunted" and the second is "at the orders of whom?"
I think the answer to the first question is "yes, probably, if they can be." We know that in the real world, when wild animals have valuable body parts, they have been hunted, even when they are considered fierce and terrifying animals. Consider whaling in the pre-modern era - a very dangerous endeavor with a high fatality rate for the crews. Whalers were, indeed, often called "adventurers". And if you have Lairs & Encounters you'll have noted that many, many monster parts have value beyond simply being worthwhile as special components. Monster fangs, claws, feathers, carcasses, armored hides, and more. Elephant tusk and whale blubber are valuable in the real world without needing magic and so I think magical special component value has to be thought of as additive to that.
The answer to the second question is "it depends". In remote areas without functioning markets, mages may hire adventurers to fetch the special components they need. In highly urbanized areas, mages who function in guilds in urban settlements may well contract with the merchant's guild, who in turn hires adventurers to fetch the special components they need.
All of the above is, of course, subject to Autarch's first law, that every campaign is a law unto itself.