I know it's a wide question so anything that is literally junk we can discount for the purposes of this poll.
I tend to keep boxes for things I use for this specific reason. A lot of my "used" items are in good condition so it makes sense to keep the box and maximize any money I can get for them.
Just wondered how many people can be bothered and how they do it.
We sell stuff it is worth selling. Craigslist for big stuff and Amazon Marketplace for small stuff. A lot of "junk" isn't worth the time, so we just either donate it to GoodWill or throw it away.