"How do I list relist multiple items all at once"
If you use Selling Manager, go to your Unsold page.
Select (click the box) items you want to relist
Click on "relist"
The next page will show all items you have selected. You may relist "as is" or make changes, as you wish - either individually or for all of them.
If you do not use Selling Manager, you should. For more information:
http://pages.ebay.ca/help/sell/sellingmanager.html