What are the implications of "Ki Avadai Heim"?
Rashi: It is a reminder that Hashem's document (that we are His Avadim) precedes the Sh'tar of sale that the Eved Ivri handed his master.
Seforno: Even though the Eved purchased a master for himself, and therefore deserves to perform whatever tasks are placed upon him, due to the fact that he is My Eved, he does not have the authority to sell himself as a slave - unconditionally.
Having already said "Lo Sa'avod bo Avodas Aved" (in Pasuk 39), why does it repeat "Lo Yimachru Mimkeres Aved"?
Rashi, Targum Onkelos and Targum Yonasan: It is a prohibition against selling an Eved Ivri in the way that one sells an Eved Cana'ani - by announcing his sale or by placing him on the 'stone of sale'.