Unfortunately the housing crisis sort of proves this isn't the case in the real world, given the option of 10 valuable items or 10,000 valuable items people choose the 10,000 because they don't consider what flooding the market would do to the price...
There's also the fact that they're travelling merchants, they probably aren't just going mountainhomes -> you ->mountainhomes. Sell a few bins of trendy turtle shell trinkets to the mountainhome and then sell some more to the humans and more to the elves. That offsets the diminishing returns.
That is much better than my explanation
That does make sense, but I still think there should be a point where the trader simply doesn't want any more of the same crap.
I mentioned the mountainhomes specifically only because that's a place I can think of off the top of my head where people would be interested in buying crafts just for the sake of having them. I'm sure there are other places out in the world that would like to import the crafts I make, but I'm also sure I've exported so many by now that they could give a free ≡mussel shell figurine≡ to every customer and still not be rid of them all. Plus, not every location the merchant visits is going to want them - in my fort (and I'm sure this holds true for many players) I never buy crafts unless I'm planning to melt them down for their metal.
Similarly, if you're trying to buy ALL the trader's food or steel or leather, the price should start going up after a certain threshold. Both because you're buying out his stock that he might have sold for a profit at other settlements and - more importantly - you're demonstrating that you want it badly enough that you're probably willing to pay more for it.
Another thing to keep in mind is that the mountainhome presumably isn't the same as a fortress, and the other places the merchants go (with the dwarves acting of their own volition rather than under player influence) may not want the same things that an all-seeing overseer would consider valuable,like how dwarves themselves consider murder terrible, but some overseers will atom-smash a dwarf for complaining that the water tastes funny. So while iron bars and leather may seem like invaluable materials to you, other places might think "That stuff's just trash to shove onto the caravans to haul away, but damn if only we could convince those caravans to bring some decent food and trap components, and maybe some toys to get these kids to stop bugging us". And while the "market" doesn't really exist yet you can encourage the caravans to bring stuff by agreeing to pay more for it when the liaison comes by...
(mostly playing devil's advocate here, you're probably right about the supply and demand thing not making sense, but a whole world economy is pretty complicated to put in when a lot of people are still getting adjusted to all the changes from DF2012)
That said, I still love the mental image of the king/queen receiving my offerings and responding "What the hell am I supposed to do with 5000 pond turtle shell earrings?"