I don't think the individual reselling sunglasses is necessarily being "unethical". As you point out, there's nothing obviously wrong with the individual actions.
The reason it's troubling is that they are making money without actually adding any value. The sunglasses were created by factory workers in China. If you sell a pair for 100$ and they only get a few pennies, they are being ripped off.
Capitalism doesn't reward creation of actual value. It mostly rewards ownership of capital[1], and occasionally 'hacks' like the OP's. Honestly I am not too worried about them making a few thousand dollars from people with too much money. But it's the tip of the iceberg w.r.t. a dysfunctional economic system, which is why it tends to rub people, especially those who try to 'make an honest living' (by e.g. actually building something useful), the wrong way.
If factory workers are getting pennies while other people in the chain make orders of magnitude more per sale, that's a price signal saying you shouldn't take a factory job becasue other roles are more needed. If you mess with those motivators you'll just end up with a less efficient system as yet more people pile into such jobs when we need them to learn skills and contribute elsewhere.
You're thinking in terms of a steady-state. If a middle-man is able to do such a high markup then it means that yes we need more middle-men, because the increased competition will reduce their ability to have such markups. The need is proven by the high margin itself. If we add more factories and factory workers, on the other hand, end buyers won't gain squat since you're only addressing a tiny component of the cost.
And when factory workers try to unionize, and the company spends millions of dollars on anti-union propaganda, that's just price signals, too, I suppose?
No. That is the owners of capital wielding their power to keep the workers--the foundation that keeps the whole show running--underpaid, so that the owners can maximize their profits. Capitalism 101. Maybe actually read the book I linked to. 7 billion people in this world don't live in the free market fantasy land that you're describing.
The reason it's troubling is that they are making money without actually adding any value. The sunglasses were created by factory workers in China. If you sell a pair for 100$ and they only get a few pennies, they are being ripped off.
Capitalism doesn't reward creation of actual value. It mostly rewards ownership of capital[1], and occasionally 'hacks' like the OP's. Honestly I am not too worried about them making a few thousand dollars from people with too much money. But it's the tip of the iceberg w.r.t. a dysfunctional economic system, which is why it tends to rub people, especially those who try to 'make an honest living' (by e.g. actually building something useful), the wrong way.
[1] Lenin wrote about this back in 1916. Read Imperialism and tell me it hasn't gotten ever worse since then. https://www.marxists.org/archive/lenin/works/1916/imp-hsc/ch...