In times of war it becomes very difficult to find basic goods such as food items, or hygiene supplies. Some people will get out of their way to collect such goods and make them available to the general population. Clearly, those goods must be sold at a very high markup, to cover not only the expense of buying them but also the personal risk of the merchant to procure and sell them. Clearly it's more beneficial for th…
If the demand and supply elasticities are really totally inelastic there's nothing to be gained from allowing market activity. No new supplies will become available due to a high price and no one will cut back on their consumption either. You'll just be allowing folks to get rinsed financially at their lowest point which of course sucks.
If either elasticity isn't perfectly inelastic then there's scope for increased efficiency through market activity, at the cost of some or lots of folks getting rinsed financially. That's the tradeoff and usually I'm for allowing the rinsing and making up the inequity on the backend in order to squeeze every last bit of efficiency out in an emergency. But I'm not die-hard committed to the idea and recognize the tradeoff.
The last point to mention is budget constraints. If Bill Gates is around and has 1000x more money than I have he really can inefficiently hoard resources even if he's not perfectly inelastic in his demand. This is a trickier situation to justify black markets but I think it's so vivid and awful that folks' minds jump right to this situation when not all emergencies are like this.