According to WGME, when undelivered mail is declared "dead" it can end up being auctioned off for profit.

If, for some reason, your mail cannot be delivered, it ends up in a government facility called the Mail Recovery Center.  At the MRC, kind of a lost and found for the postal service, mail is opened and inspected for clues that might lead postal workers to the intended recipients or the sender of the mail.  If the owners cannot be found, the mail goes up for auction.

Those auctions are run by a private company called GovDeals.  Instead of attempting to purchase individual items, the bidders bid on large lots of items in boxes.  In many cases, you only see the top of the box.  There could be some real treasure buried inside, or Aunt Edna's garden gnome collection.  Actually, they do seem to be separated by type (auto accessories, health and beauty, video games and accessories).

We don't really know how much mail ends up being taken to, and auctioned from, GovDeals "secret" facility in Atlanta.  But, in 2014, 88 million pieces of mail were brought to the MRC with only 2.5 million eventually being delivered to their rightful owners.