When a listing changes from an Auction to a normal sale listing, the auction date remains saved against the property. The solution to this is to delete the auction time/date when you are changing back to a normal sale listing. To do this retrospectively:
Click into the listing, then go to the Listing Details
Change the Authority field back to Auction (don't click save)
Remove the Auction date by clicking the 'x' button under the date
Change the Authority field back to the previous selection
Click Save at the bottom