Can an author purchase book returns from the publisher? (I'm thinking of trade paperbacks.) What percentage of the original price would the author expect to pay? Could the author sell the books? Use books as door prizes? Give books to people doing reviews, to people winning contests and other promotional uses? It seems like this would help both the publisher and the author and make it easier to promote the book.
Typically, there is a clause in your contract with a publisher stating that you can buy copies of your own book at a discount of 40 or 50%. You can do this at any time, from day one.
There is no standard clause for buying returned copies. It can be complicated. Some publishers may choose to destroy returns--it might be cheaper. In other cases, publishers may want to keep them on hand and see if they sell down the road. You may get most of your books returned, but then sell them all (and go back to press) a year later. So I don't see publishers rushing to sell these at an even deeper discount.
That said, you can indeed buy books at a very deep discount if they are REMAINDERED by the publisher. Sometimes, thousands of them, for as little as $1 or $2 a book. Once you own them, you can do anything you wish with them.