Since the zero government society (ZGS) will have nobody to write or enforce laws, at first sight one might be worried that the practice will proliferate. It won't; the market will work, as always.
The market elegantly solves three acute problems: (a) some girls get pregnant who do not wish to give birth, (b) some people have a religious or other belief that abortion is wrong, and (c) some couples would dearly like to have a child, but cannot. All that is a situation the market almost begs to resolve.
Notice that a born baby is a human being, and therefore a self-owner, and therefore is not property, fit for sale or purchase; any such transaction would be void. Therefore, "baby-selling" is not a term that can be used in this context. Rather, what will evolve will be a contract between the mother and the adopting parents, without interference from a third party such as government. The contract will specify the rights and responsibilities of each party, and will be binding; and there will not be a single form of words for everyone to use. The particular parties will compose whatever words they want.
For example, the contract might provide that the birth mother shall retain the right to reclaim the baby (reverse the transaction, return any money paid her) within a certain time period. Or, it might deny her any such right, from the moment she signs the agreement. The agreed price will depend in part on which of those options is chosen, for the adopting parents will be eager to be sure that once the deal is done, it's done. So the average price for the latter will exceed that for the former.
The contract will relate to parental duties and rights. The birth mother will give up both, but gain some money; the adopters will pay the money and accept the job of parenting. It's what they desire. The wording may well also refer to timing; a broker or adoption agent may well help bring the contracting parties together, but will impose no delay such as results from waiting periods dictated by decrees.
As a result, the baby will gain the enormous advantage of being brought up by parents who desire him or her, as a treasured member of their family. The market will thereby arrange that the tragedy of unwanted children will end, or become insignificant. At the same time, it will appease those who fervently believe that to abort a fetus is morally wrong.
Thus the whole, tedious, passionate controversy about whether one law should favor one viewpoint or the other will terminate at long last; Roe v Wade will be history. Those actually involved in each real situation will agree among themselves exactly what they wish to do.
As the child grows more mature, his or her self-ownership right will kick in to ever more practical effect, and the parents will, as always, have to let go. Thus the adopters, like all parents, do not "own" their baby, and never did. What they are buying from the birth mother is the priceless experience of raising him to adulthood.
Some may shriek: but what about trafficking? Would not freedom like that lead to pimps buying babies and raising them to become beggars or whores in a state of slavery, like those villains in the Indian movie "Slumdog Millionaire"?
Let's look at that possibility, bearing in mind the interests of the prospective pimp.
First off, if the contract with the birth mother has wording that shows the transaction purports to be the sale of a baby, it will be void and reversible on its face, as in the fourth paragraph above.
Second, if the wording is kosher but someone notices that the adopter (the pimp) is not fulfilling properly the role of a parent, anyone (a charity for preventing cruelty to children, for example) could intervene on the child's behalf and obtain a free-market court's order to remove him or her to proper care. So the pimp would be out a bunch of money, yet be deprived of his prospective profit source.
Thirdly, as the child grows she will begin to exercise her right of self-ownership (which was always hers, from birth) and might very well apply by herself for a genuine pair of parents; by walking in to an adoption agency or even by advertising directly: BRIGHT, AFFECTIONATE TEN-YEAR-OLD SEEKS CARING PARENTS. That's an action any child will be able to take at any time, and so will form a powerful incentive for all parents to do a proper job.
Fourthly, it's a highly hazardous business model anyway. All babies are cute, but some teenagers are much less attractive than others; so he is investing a lot of money for fifteen years, without a clearly predictable revenue stream then resulting. And if he resorts to drug dependency to keep the teens from escaping, their viability as hookers will be severely curtailed.
And finally, as soon as it becomes public knowledge that the pimp is running this kind of scam, his reputation (which will be everyone's primary asset) will fall off a cliff, with or without judgments against him by free-market courts; so much so that he may be shunned to the extent of becoming unable to buy even the necessities of life.
Like other forms of krime, I can't promise this practice will never happen. But these are five reasons why in the coming ZGS it's very unlikely to prosper.