Voluntary tax is charity. The whole point of tax is that it applies to everyone by some set of rules that democracy has arrived at.
Taxes work because we can (imperfectly) enforce them. What happens when we can't?
Or is the voluntariness of the "customer relationship" one way?...in which case you would really just be arguing for a global decrease in immigration restrictions.
The fundemental difference between a citizen and a customer is that it's much harder to citizenship status.
For example if we added market dynamics to a wealth tax we could make it viable and fair for all participants. Lets say there is a 7% tax on all assets over 25k USD. You decide the real value of the assets but the rule is they must be on public auction at all times. Most homeowners would price their homes above the market's rate so they are not at risk of being bought out. Large land holders would price close to the market price or below to avoid the tax. Governments could buy up large tracts of land at once without the common problems of putting in the first stake and the next home being worth 3x what it was yesterday. The end result is a truth on assets you couldn't get from central regulation (good luck avoiding taxes when it gets bought by someone else for cheap).
All of these ignore the pragmatics of whether or not people want their homes auctioned 24/7 but the dynamics of the system are better and self regulating from a taxation perspective. That's the thing about these ideas is of course they go against platonist sensibilities that things remain more or less the same forever. The dynamics change so it should be in everyones interest to explore alternatives that don't just assume people will remain as a doscile tax base until we all perish to dust.
To answer you more directly: citizenship as it stands also has a barrier to entry. I'm trying to make the government more reactive, and give people more power to decide what society should spend its money on. I'm not so certain everyone would just vote to stop protecting those around them, we all know someone who needs help.
1) Markets aren't magic coordinators, they take large amounts of human effort to maintain in a functional state. Your system would destroy productivity because it would require a huge increase in market oarticipatio
2) Economic activity needs stability and your system would almost entirely eliminate that.
3) Markets require information to function accurately so to constantly have accurate markets for everything means that you have completely eliminated privacy.
4) Price is not the only important aspect is many transactions. Reputation matters and this system would result in the inability to trust any economic partner because their assets could be bought at any time.
5) You would still need to have massive government regulation to prevent fraudulent collusion. Lets say you seriously underprice your assets to avoid taxes and whenever someone moves to buy the asset, you have a friend who would bid on it until they win. That friend can then sell the asser back to you. Now raise this basic idea by the power of the massive legal and financial efforts that have histoy been put into tax evasion and you have a regulatory problem that dwarfs existing tax enforement problems.
So you would basically tax stability while creating tools that would enable massive fraud and speculation.