XII Tabularum? What is that, the "Twelve of Tables?" I've dispatched a Jesuit Latin teacher to your location to rap your knuckles for using the genitive instead of the nominative. Alternatively you could write "XII Tabulae" a hundred times before sunrise, at which point I'll dispatch John Cleese to cut your balls off.

The Twelve Tables mention the sale of a son by his father, but the Twelve Tables are an archaic legal code and by the historical period this had lapsed. I do not know off the top of my head of a single recorded instance in which mancipatio of a son (the type of sale referred to) ever actually occurred. It's doubtful whether even at the time of the creation of the Twelve Tables the sale of a son by his father still happened, since the Twelve Tables offer an alternate version of a different form of bondage, nexum or debt-bondage, that involves a symbolic rather than real sale. The symbolic rather than actual sale that existed for sons (see below) in the historical period evidently existed even when the Twelve Tables were written. In any case nexum ceased to exist in 326 with the lex Poetelia, and other forms of free personal sale certainly ceased to exist before that, if they had ever actually existed in any systematic way at all. What the Romans of the historical period understood the Twelve Tables as describing here is not the literal sale of sons into slavery, but the basis for the ritual of emancipatio. Sons remained in the potestas of their fathers, and they could only be released from this potestas by the death of the pater familias or by emancipatio. Emancipatio involved a ritual "sale" of the son to another pater familias in triplicate, at which point the son was freed from his father's potestas and became sui iuris while his father was still alive. The logic is that patria potestas over a son is not quite analogous to a master' power over a slave, even though the Twelve Tables at least preserve the possibility, whether it was ever a reality in practice, of a father's selling his son. Instead patria potestas was a form of power over a free person, and thus simple sale alone could not alienate that power as it would ownership of a form of property. Thus the provision in the Twelve Tables, and the repetition of the symbolic sales.

Emancipatio does not, however, appear to have been particularly common. In the first place, it served very little purpose. The father who agreed to emancipate his son likely wasn't going to abuse his power as pater familias, and in any case I'm not aware of any instance of emancipatio being used to separate a father from his son either as a punishment against the son from the father or as a means for a son to escape the exploitation of his father's potestas. Being in potestate did not prevent military duties, running for public office, or admission into the senate (by the late Republic requiring election to the quaestorship in any case), although there was a handful of things that men who were not sui iuris could not participate in. As I recall they couldn't be flamines and stuff like that. Emancipatio also screwed with inheritance law, since technically an emancipated son was now broken from the normal succession. He could still, of course, be included in wills, and a praetorian edict later wrote in a sort of exemption for emancipated sons so that they could still inherit automatically if his father died intestate. Since an emancipated son lost agnatio with his father and his father's entire line emancipatio formed the basis of one form of adoption, the other being arrogatio, which only worked with men who were sui iuris. In adoptio proper only a son in potestate could be transferred, and the mechanism used was the same symbolic triplicate sale, just without the resale back to the original father--thus agnatio was transferred to a new pater familias. But that also means that emancipatio was not compatible with adoptio, and arrogatio likely happened almost exclusively when the man in question was sui iuris not because he had been emancipated but because his father was dead.