@Roadrunner9000 thats the thing: its rare, nobody else supplies it, the kingdom would pay the price he asks for. If they say 'no, their value is 1G' like your version he could just say 'well, then Im not selling' and they couldnt do anything, and the kingdom who has interest on said product would be left with nothing. Thats basic market, supply and demand, the way things are done everywhere (even on this manga, remember different potions have different values and he spent a lot of money on them).
Traveler merchants have no need to supply anymore than what they have on them as well, you dont need to make a full market out of a single trade. "Here I have this, if you want it costs X", the end. You make it sound like hes gotta work as if he was a big company with a set contracted supply, which makes no sense.