Warringah Council has kindly offered the following comments (in blue) relating to garage sales.
 
(If there is no blue answer on a particular question, then a comment wasn't made.)
 

• Are permits required? Council does not require approval to hold a garage sale however garage sales should be conducted as an occasional, low-impact activity.
• Are there any laws controlling the posting of garage sale signs? Signage is not allowed on public land, including footpaths.
• Are there limits to how many garage sales one person can hold per year?
• Are there laws about using the footpath in front of the person's house? Items for sale are not allowed on public land, including footpaths.
• Other comments offered. Warringah Council encourages recycling of unwanted household items rather than throwing them out in Council's general cleanup, which then ends up as landfill. Council supports the Second Hand Saturday program.
• Is there a page on the web site that covers the laws relating to garage sales? No.

Disclaimer:
1) Laws are always subject to change.
2) If there is any doubt, you need to confirm current council laws.
3) egaragesales.com.au provides this information in good faith, but will not be held liable or responsible for any losses or penalties incurred as a result of changes to, or misinterpretation of, current council laws as amended from time to time.