This is not the whole story, by any means.
Those who buy it are those who can afford it. They also (think) they need it, for some reason. That can include the intention to legally resell it. Buying suddenly comes with even more time pressure. Buying now becomes both a hedge and speculation, with zero legal risks.
Sure, people bringing in supplies on their own sounds good. It can even be good. But how much does it actually bring in? Does it come with other problems, like additional stress on infrastructure? If this really is such a great thing, I don't see how it should be wholly incompatible with anti-price gouging laws. If you want to specifically incentivize private transport of goods from out of the area, you don't have to allow them to be sold for any price, nor do you have to allow it for goods already in the area.
As for the black market, I'm not sure that's a bug. It makes it harder, and adds some risk, to acquiring more. If you actually need it enough, you'll do it. There some be some correlation between actual need and willingness to participate in the black market. With proper use of the discretion available to law enforcement, maybe this even enhances how well the goods end up distributed. That's a big "with" but still.