Seeing as we are obligated to kill all members of the seven Kena'ani nations and we are not permitted to subjugate a fellow-Jew, how will we do tasks that one may not command an Eved Ivri to do?
Rashi: Hence the current Pasuk permits making use of the surrounding nations, who are not Kena'anim, and whom one is therefore permitted to purchase as slaves.
"Mehem" is extra. What do we learn from it?
Moshav Zekenim citing Gitin 38a: You may buy Nochrim (Kinyan ha'Guf, i.e. they become your property), but they may not buy you (they can acquire an Eved Ivri only for his labor), or one another.