I've never had a garage sale but I do sell things on eBay and on Gumtree. I don't make much money because I sell odd bits and pieces I no longer use. I find that Gumtree is easier to work with and, unlike eBay, does not charge a fee when an item is sold. If I can't be bothered posting an item online, or an item does not sell, I usually take it to a Vinnies shopfront.
I'm the same re Gumtree and have a system where I drop the price by $5 after a few months of having to renew them each month. Then I'll make them free, a lot goes then and only after that, of no views/takers do i take them to an op shop if the item is worth that.
I'm constantly getting new stuff in to list (finding things in hard rubbish piles in my junk mail rounds) and have limited space.
I did try a garage sale last year in the Big Garage Sale weekend but it wasn't that successful for me because I'm down the end of a block of units and no one can see it from the street, despite me putting out signs.