Unvoiding retailers in Inzant Sales is only possible from the Management Portal / Web Client. You'll need Admin user privileges to be able to login and perform edits of this nature.
Once logged in, navigate to the retailer search screen. Click the "Retailers" icon in the top menu, then in the retailer widget click the top heading zone to open the search screen.
Once the search screen is open you will need to check the "Include Void" checkbox along the bottom of the screen. Once this is on, then double click on the retailer from the list to open the details screen. If the retailer list is quite long you can cycle through the pages to find the retailer, or better yet simply do a search to find the retailer quickly. All void retailers should have a small orange exclamation mark icon next to them.
Once you've opened the details screen, to Unvoid the retailer click the green tick icon in the top right of the screen.
This will cause the page to close and return you to the Retailer Search screen. All Done!