All returns from Costco get sent to the compactor. A couple years ago I took a load of aluminum to Las Vegas to recycle. Sitting in the yard was about twenty or so pallets of random items. Chainsaws, ladders, bikes, you name it it was there. All scrapped for minor issues. The chainsaw was missing the gas cap. Four or more pallets of ladders were slightly damaged from shipping. Only one stack of ladders were messed up. The rest were perfect. The society we live in is ruled by gimme gimme gimme. Its broke, GO GET A NEW ONE!!! Fixing things is out of the question, or mental range of most people.
Costco is a little different, they basically break even on items after their overhead, they profit mostly on their food (fresh meat/produce etc, the restaurant at the front) and membership fees. gas it has gone both ways for them in the past.
When you're talking about a single store returning half a truckload of ladders, they may not want to pay the shipping or the vendor won't take items back (a lot of the bulk non-grocery items they don't) seasonal stuff is basically all destroyed like you said. If you've been in a costco, you know what i mean by 'the entire middle of the store' is sell it or trash it except for books & clothes.
I won't argue that HD might not have the consignment contracts anymore, but I know of at least 2 Lowe's stores near me that currently do. Sears too, though I think theirs is a little more complex and probably a big sears internal thing.