I guess it really comes down to how you conceptualize the nation-state. I think most people view a nation as a community, a body politic, maybe even an extended family. Buying citizenship is just as tawdry as paying for a mail-order bride.
But I’m sorry. Me and many others see the nation state as a service provider. An institution that enforces the rule of law, furnishes protection from invaders and criminals, and provides large-scale public goods in exchange for some slice of tax revenue and a code of conduct.
But if I’m unhappy with a service provider, then I should be free to change vendors to whomever meets my need. No different than the freedom to pay for a new phone plan or new gym membership.
You complain that ordinary people don’t have the same consumer choice as the ultra-wealthy. If so, the focus should be on how to make the global market for nation-states more competitive for a wider range of consumers. I agree that many are locked into a shitty service provider. But if anything, the tiny slice of people who can exercise a choice almost certainly makes this market more, not less, competitive.