Rashi on Leviticus 25:42/רש"י על ויקרא כ״ה:מ״ב

1כי עבדי הם FOR THEY ARE MY SERVANTS — My document (deed of purchase) is of an earlier date ( Sifra, Behar, Section 6 1).

2לא ימכרו ממכרת עבד THEY SHALL NOT BE SOLD AS THOUGH BY THE SALE OF A BONDSMAN — by public announcement: “Here is a slave to be sold!” Nor shall one make him stand on the auction-stone (Sifra, Behar, Section 6 1).