As G.A. Cohen writes (in Self-Ownership, Freedom, and Equality, p.25):
Among the reasons for limiting how much an individual may hold, regardless of how he came to hold it, is to prevent him from acquiring, through his holdings, an unacceptable amount of power over others: the Chamberlain transaction looks less harmless when we focus on that consideration.
Nozick assumes that someone willing to pay 25 cents to see Wilt play is ipso facto willing to pay 25c to Wilt to see him play. But Cohen points out that this assumption is illegitimate. In an egalitarian society, the vast majority of citizens might well deem the resulting inequalities and potential class divisions to not be worth it. They might voluntarily choose to preserve the patterned distribution that they value, and avoid transactions that threaten it.
This gives rise to a collective action problem: it might be that each individual prefers to preserve equality, but doesn't want to boycott the game without some guarantee that enough others will do likewise. One way to resolve this problem could be a "democratically authorized taxation system which maintains wealth differentials within acceptable limits." (Cohen, p.26.) Individuals could then freely exchange goods, safe in the knowledge that they won't be risking their deeply-valued social equality in doing so.
But suppose the fans care more about basketball than equality, and so are willing to pay Chamberlain in order to see him play. Do we have any grounds to oppose this voluntary exchange? Nozick claims that third parties to the transfer are not affected - "their shares have not changed". But, in an important sense, this simply isn't true:
For a person's effective share depends on what he can do with what he has, and that depends not only on how much he has but on what others have and on how what others have is distributed. If it is distributed equally among them he will often be better placed than if some have especially large shares. Third parties, including the as yet unborn, may therefore have an interest against the contract. (Cohen, pp.26-27)
For a clear example of this, even Nozick admits that an individual may not purchase all the drinkable water in the world (ASU, p.179). If all the water-holes in a desert dry up but for one, the lucky owner may not charge exorbitant prices to the dehydrated locals (p.180).
Thus the state may legitimately interfere in voluntary exchanges so as to prevent accumulation of wealth and power that risks making many subservient to a privileged few.