From GarageSaleWiki

If you need to modify a listing after you started it, you can revise your listing inside GarageSale.

To do so, switch to GarageSale's template mode or in GargeSale 4 use the Inspector and...

1. Select the auction template you started your listing from. 

2. Make the changes you want to incorporate into your listing to the selected template. 

When you are done, go to the templates 'Auction' section by clicking the switch control at the top right, (NOT the mode buttons in the top middle!).
In GarageSale 4, you simply have to use the Inspector (tab Auctions).

3. From the auctions table, select the auction to revise and click the revise button.

4. You will have to choose the account if you have more than one.

5. GarageSale will then revise your auction...

Note, there might be several reasons why eBay might not accept your modifications!!!
In this case, of course, GarageSale will warn you:

Revising Auctions prior to GarageSale 2.3

Change your auctions's attributes as desired and select Revise Last Auction… from the Template Menu. If eBay approves your changes, GarageSale will switch over to auction mode and you should see your updated listing.