ἐστὶ γνωρίζειν goes with the previous bit:

περὶ τοιούτων τινῶν εἰσιν ἃ κοινὰ τρόπον τινὰ ἁπάντων ἐστὶ γνωρίζειν

"they are about such things as it is possible to become acquainted with [as being] in a certain manner common to all"

Then οὐδεμιᾶς ἐπιστήμης ἀφωρισμένης must be a genitive absolute: "with no system of knowledge being distinct / specifically dedicated [to them]".

ἐστὶ γνωρίζειν goes with the previous bit:

περὶ τοιούτων τινῶν εἰσιν ἃ κοινὰ τρόπον τινὰ ἁπάντων ἐστὶ γνωρίζειν

"they are about such things as it is possible to become acquainted with [as being] in a certain manner common to all"

Then οὐδεμιᾶς ἐπιστήμης ἀφωρισμένης may be a genitive absolute: "with no system of knowledge being distinct / specifically dedicated [to them]". Or perhaps better, as brianpck suggests in comments, it can be taken as parallel to ἁπάντων, "common to all and not to any distinct system of knowledge".