From GarageSaleWiki

Finished auctions that haven't received any bids can be started again without paying listing fees twice. eBay will credit you the listing fee for your original auction after your item has been sold. This procedure is called 'Relisting'. To relist an from within GarageSale...

Select the auction template you originally used to start the auction (in template mode).

Go to the auctions section of the template (on the right).

In the list of auctions started from the template...

Select the auction you want to relist and click on the Relist button. 

A dialog will appear where you can choose if you want to relist the auction unchanged or if you want to use the changes you made to the template since you originally started the auction.

If you wish to relist your auction regardless of the result, consider restarting your auction automatically (new in version 3.1).

Relisting Auctions prior to GarageSale 2.3

You can relist auctions that haven't received any bids by switching to GarageSale's auctions mode and choosing Relist Auction… from the Auction Menu. eBay will credit you the listing fee for your original auction after your item has been sold.