Is there a single word (or a short expression) for selling something (usually online, like on craigslist etc., also possibly garage sales) that you don't use anymore?
You haven't provided sufficient context to give you much of an answer beyond generic phrases like selling used or pre-owned or second-hand items.
You could be getting rid of junk you don't want. You could be selling items that some people would consider rare finds. You could be selling items that still have lots of life and use left that shouldn't go to waste.
I don't know of any one specific word for selling a single item. Something close to what you are thinking of might be the phrase 'declutter.' If a family was going to hold a garage sale to get rid of things they really don't need anymore they might say they are going to 'declutter' the house. As in "get rid of the unneeded clutter."
I have updated my answer.
For the actual act of selling, and using as few words as possible:
"Selling your cast-offs."
The noun cast-off (as opposed to the adjective obsolete):
Something, especially a garment, that is no longer wanted.
‘I'm not going out in her cast-offs!’
While the act of selling something can have different descriptions (and you would need to clarify the intent behind it), the thing itself could be considered obsolete:
a : no longer in use or no longer useful · an obsolete word
b : of a kind or style no longer current : old-fashioned · an obsolete technology · farming methods that are now obsolete
This is not to say that if something is obsolete for you it is necessarily obsolete for somebody else. (Hence the saying "one person's junk is another person's treasure.")