Brisbane City 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? No.
• Are there any laws controlling the posting of garage sale signs? Signs are not permitted in the CBD or on road reserves and public areas including light and electricity poles. You can place an A-Frame style of sign outside your property but only on the day of the sale similar to a real estate agent open house inspection. You may display a sign within your property boundary. A suggestion offered is that, with permission from the homeowner, you may be able to place a sign within the boundary of a corner property - but you must have permission.
• Are there limits to how many garage sales one person can hold per year? Council has no set figures on number of sales per year. The critera would be that the goods sold must be unwanted household goods - not goods brought in specifically to sell (which would be considered a business operation). If regular sales draw the attention of council, each case will be considered on its merits.
• Are there laws about using the footpath in front of the person's house? The garage sale must be held within the boundaries of the householders property.
• Other comments offered. It should be noted that Council Officers patrol 7 days per week and will look at any issues which appear to fall outside the appearance of a garage sale.
• Is there a page on the web site that covers the laws relating to garage sales?

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.