Businesses will also sell old stock at a loss simply to free up capital that's trapped in stockholding.
Not to mention that often you're charged a restocking fee by your distributor and it counts against your account purchase totals (which grant you perks such as line discounts, better rates overall, etc).
When I was running my business our main distributor would allow a 6 month restock, but with a 15% fee. We'd simply put inventory that wasn't moving that we would have returned on "clearance" at (or up to the 15% below) cost to get rid of it. It was only things that we literally thought we'd never get rid of that we'd return.