I’ve been looking around for more information on these mysterious chunks of Greek, found in PG 12 and PG13. Migne is really very vague about the origins of this material, and it isn’t even mentioned in Quasten. However at the start of PG 12, where the biblical materials of Origen begin, there is a praefatio (col. 9), which looks relevant.
The second volume of the works of Origen includes many fragments of his exegetical works on Genesis, Exodus, Leviticus, …, partly already printed, some made public for the first time. We have edited whatever remains of the Latin version by Rufinus faithfully from the old manuscripts. But we have added the Greek fragments in this edition … whatever is collected in the Greek Catenas under the name of Origen. I have looked at all the fragments ascribed to him, whether those of Combefis from Paris mss, or those which Ernest Grabius copied from English manuscripts. Those were kindly communicated to me by the learned Fr. Louis de Touremine, SJ … ; but these were transmitted to me by the learned English doctors Walker and Bentley. I have also seen the fragments which in many places appear in the various Greek Catenae of the Fathers, which were published by Corderius, Barbarus, Ghislerius, Comitolus, Patritius Junius, and others. But the accuracy of everything in the catenas is uncertain… the names of the writer of the same fragment is given in one catena as Origen, in another as Didymus, or Eusebius, or Theodoret, or others. … [he uses his judgement as to what to include].
So it looks as if the Selecta are essentially extracts from the catenas. Each extract in a catena relates to a specific bible verse; so the editor has merely compiled these, for each work, extract by extract, in chapter/verse order. There seem to be Selecta printed for each of the homilies of Origen.
The title page of PG 12 tells me that the works are edited by Charles and Charles Vincent de la Rue, priests and monks of the Benedictine congregation of St. Maur. So Migne is merely reprinting the Maurist edition, it seems; yours, according to the title page, for 15 francs.