And the price the market will bear is proportional to the profits needed for a company to be viable. If wholesale prices doubled next year for all home improvement goods, do you think that home improvement stores would cease to exist? Or would the retail prices for everybody increase significantly?
If the cost of doing business goes up for one business and not others, they will face competition and either have to find a way to sell their products at the new price (maybe capitalizing on a niche in quality or location), or they will go out of business because all consumers are price-sensitive enough that if an equal alternative is found at the same price, and they have that information, they will choose it. If the cost of an entire industry goes up, demand goes down (at a rate dependent on how price-insensitive the consumers are), so businesses have to select whether to raise prices or eat the loss of their margins. Some may, in fact, go out of business if they were operating too close to the red. Not every company is entitled to operate a booming business, and they should not expect to be unaffected by cost changes or price sensitivity in consumers. A consumer is not now ethically obliged to pay the higher price.
Relatedly, a business right next to or on a military base, where most consumers can be expected to have military credentials, will necessarily have to operate differently than one away from a military base. Same for businesses on a university campus. You have to operate in a way that makes sense for your location and your clientele.
That's not how ethics works?
The question, to me, seems like "Is it unethical to agree on a transaction with a merchant based on fraudulent information?"
Sure, ethics has different frames. I've never heard of anyone reasonably claiming there is only one way to approach an ethical dilemma. My framing of the ethical quandary in this scenario is not yours, and I can understand your framing, but it's not the one I most identify with. I do not see the transaction occurring with fraudulent information, I see it more as a gift from a family member. The commissary/exchange/etc. allow gifting items purchased at a discount, just like the tax code allows for $15,000 annually of gifting. This is reasonable human behavior. There are rules, however, in an attempt to limit this to legitimate gifting and not a business-in-disguise or other form of tax/price avoidance. (I mean, when we lived on a military base, I thought nothing of having a friend who came over pay for their own snacks at the commissary or even going shopping for larger ticket items at the exchange, even though they are "benefitting" from the tax-free nature of the store. They are my guest. It is reasonable to go shopping as an activity near my home.)
In another lens, if a military vet bought a couch, decided they didn't want it, and sold it to somebody at the price they bought it for, nobody would blink an eye. If, instead, they bought out a furniture store at their discounted rate and sold everything for 5% higher on craigslist... to be honest I'd question their sanity, because that sounds like an awful lot of work for very little gain, and it would probably not be successful anyway. But things that are stupid are not necessarily unethical, so as long as the consumer is operating within the terms of the discount (not every store with a military discount will have rules about what happens after the purchase) I consider the ethical perspective moot. It's a business operation, not a charity.