The best way for market would be to keep the NPC packages high so that it remains an expensive way to get stuff fast and with no need to search for supplier. But arbitrating prices manually to keep packages expensive and not too far overpriced would require a lot of work.
So the way to go might be to add some costs to the seller. The most obvious thing is limiting NPC package daily sales number. Then they will strive to keep the prices as high as they can to still sell that number of packages, but not to dump as many of them as they can. The less obvious thing would be to require some rare resource to “fuel” the package sales. This could be more interesting, but seems harder to think through and implement. At first glance (nb: it’s a rough idea with some obvious flaws), you can’t sell gold in NPC shop (I hope, at least), so if every sale used a set amount of gold from seller’s account (from OCD or some special “fuel” as with HWS autominer), that would also limit the price dumping.