Please do not answer this post if you are against government and taxes. I would like to talk about a technical solution, not about politics.
What if no one is against taxes and government? What if we are against the frameworks they operate? That makes it necessary to respond.
I think it would be better if sellers could provide a proof that they are not dodging taxes.
Providing proof and requiring proof different things. LOL.
We can even imagine a future legal framework...
register their bitcoin addresses to a government entity, and where unregistered addresses would be allowed only for private and non taxable transactions.
You haven't the slightest clue what the word Register means. Register means you accept a given institution to have authority over you. That includes suspending your constitutional rights. No sane person would register anything if they knew what the word meant. You can instead Declare and Associate an identification token and that is of your own free will without any implication for your liberty.
Prevention of unregistered addresses would be adding new authority. Why would anyone do that? Authority is not the same thing as Function. The fact that authority might be necessary to fullfil a given task does not entitle an institution to such authority. Take wiretapping for example. A situation may require warrantless wiretapping. It does not entitle law enforcement to as much as they might need no matter how much they need it.
Just because they need it to fullfil previous function doesn't imply automatic entitlement. Regardless of the situation. If they fail at their work, then so be it. Create an institution that finds another way.
I do not know if this has been proposed before.
Probably 100 times.
I think that such a feature would help us build a legal framework around bitcoin, and promote bitcoin as a threat to the current banking system, but not to governments.
Taxation is a function of banking not government. The only governments that actually have a say in taxation are county and local. Banks don't own those yet.
Second, you have no idea about the difference between function and authority. Authority isn't delegated like a privilege or a license. Authority is the assignment of power which can only be removed by someone of higher authority and only if that authority itself isn't a full transfer of power. When you accept authority, you are not assigning a responsibility. You are assigning someone over you.
When someone has authority, they have authority over you firstly, then they have authority to exercize their function. Power transfer is the biggest unreported scam in the history of liberty.
We now have as our three branches the economy, the media, and the military. Behind these structures we bankers, corporations, and government. Bankers manipulate the economy and own the media. Corporations influence media and sell technology to the military. The government suppresses the media and controls the military. The government doesn't represent the people. The lower echelon of bankers operates in corporations. The middle echelon of bankers operate in government. The lower echelon of corporations operate in media. The middle echelon of corporations operate in government. There is no independent echelon of government.
If the government is just a shell institution for bankers and corporations, how the hell do you not threaten government while threatening banks?