No one is entitled to a business model. If you can't exist without being ethically sound, then you can't exist - period. Capitalism is not a replacement for ethics.
And in any case: you can sell open-source software.
And in any case: you can sell open-source software.
No you're not. If you want the source code for the programs to run on your computer either write them yourself, choose FOSS ones, or pay someone to write one that supplies the source.
Nobody is mandating you buy a particular piece of software or run it on your computer. You're choosing to do it. If you decide you're not going to run closed source software that's your personal choice.
> ... or is paid for by our taxes.
That's moving goal posts.
Do you do that?
And, once again, you can sell open source software.
Granted, if you sign an NDA and pay extra, you may have the source. If this is the model you suggest, I think it's fair.
My question: Is it still unethical if the exchange and use of proprietary software is made only between people who fully understand the value of source code and decide to anyway?
If the person running that software asks for the source code, it would be unethical to withhold it from them. However, from the arrangement you made, it would also be unethical for the person to exercise that entitlement which they previously agreed to give up. Two unethical actions don't make a right, so you're just setting yourself up for an unethical situation all around. Better to just make it clear the source code's available on request from the beginning.