Over the economics thing? Sometimes, a different set of eyes...
And to be perfectly clear, those rules apply 100% of the time, even for mundane transactions.
It's unlikely, but if the party had found a Handy Haversack and wer trying to buy 10,000 torches, unless they were in the big city, it wouldn't be possible...and even there, it would take time, travel from shop to shop as they cleaned out one seller after another. And the guy selling the last 5 to be had would DEFINITELY be gouging them on the price.
If they tried buying a horse destined for Lord M'horvan's estate, they may not have enough money to do so, despite having just cleared out a bandit party.
And perhaps, in his domain, only members of his army are allowed to buy blackpowder...legally.