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 have sold things on ebay, but for the amount of money I got for them it wasn't worth the time it took to list them. It might be more worthwhile for larger/more expensive items. I have had a garage sale and that wasn't terribly successful either.
I sell very cheaply, to avoid too much time on questions etc, but I sell a lot. It started because I thought it wouldn't end in the landfill if they paid for it. Now I fly to Singapore for the Grand Prix on the proceeds, so it's win/win all round (I know, saving landfill, destroying the environment with excessive petrol)
I've sold on ebay/gumtree, and I've held garage sales, successfully in the sense that I got rid of the things I didn't need, but I didn't get wealthy on the cash. Another thing I also do is give hand me downs to my relatives and friends as my children outgrow their clothes. And they love receiving them.
I just show stuff that I want to get rid of to my friends whenever they're over, and if there's something that's really precious/expensive, then I sell it to them, else I simply give it away for free :)
Had a garage sale at my place once to help a friend sell his stuff. I swore I will never have a garage sale again. When we moved houses I sold most of my furniture on e bay and it was a breeze! e bay I most certainly will continue with. only to get rid of stuff. Both Garage sales and e bay wont fetch you too much money if you are selling old stuff ( selling antiques is different though)
I've used ebay, gumtree and garage sales all with varying degrees of success. I think it depends on the item and your pricing more than the place. Things I can't sell go to a thrift shop, school trash and treasure stall or I leave them outside my gate for a week. LOTS of stuff has gone that way.