1) This is the author, and these are rejected proofs or author copies. In which case, Amazon does not have the rights to sell them because they were not "returns".

2) This book has been sold once, and royalties have been paid, but it was returned, relabeled as "new" and sold again without a royalty being paid on that sale. The author and the customer have been cheated.

The thread has likely developed, but this is just going by my limited familiarity with publishing.