What are the implications of "Ki Avadai Heim"?
Rashi: It is a reminder that Hashem's document (that we are His Avadim) precedes the document of sale that the Eved Ivri gave to his master.
Seforno: Even though the Eved acquired 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: It is a prohibition against selling an Eved Ivri in the way that one sells an Eved Kena'ani - by announcing his sale or by placing him on the 'stone of sale'.