1) If there is high demand for anything some rich merchant type will be there to make sure it gets sold - if there is a profit to be had.
Yeah... but in a desperation scenario, what are the chances that same merchant won't just arrive with his military escort to occupy the place and make everyone slaves? Why exclude the 'good races'?
Going off goblin-only slaves:
Personally, I'm not really a fan of fantastic racism - it's just unecessary, unless there's this massive lore background to act as justification. You can't assume gray and gray - that's less valid than assuming black and white. That's why D&D starter modules often go black and white.
2) Encounters: If you are already hiring adventurers to get the stuff, send them after caves of Direbats with instructions that the Direbats are not to be killed. Direbat Guano has to be just as good for components, right?
If merchants are that desperate for bat poo, they can just farm it. If you say 'no merchants don't do that sorta thing, they're too buzy trading lul' - then a bunch of merchants would band together and invest in farming. Or one would. Why not? Farming and herding dire bats doesn't sound that hard. Especially if PCs are capable of actually ignoring dire bats (although I understand that what the OP is going for is low level stealth/action adventure where everyone could be rogues, but rogues still kinda suck on their own in D&D, etc., etc.).
If merchants are rare, how have village folk even heard of the word 'merchant' if there aren't that many around?
Easiest solution (and one commonly used, for good reason):
War scenario. Major kingdoms are at war, so that's why everyone (even in the home land) is desperate. If players leave town in the wrong direction
, a bunch of bugbears immediately murder them and t-bag the corpses.