Deleting a listing does not delete the user who added it. The WordPress user account stays, with its username and email address, so signing up again with the same details fails with “This username is already registered” or a similar message about the email.
If the person wants to come back
They should log in with their existing account instead of registering again, using the lost password link if they have forgotten it. Listings belong to user accounts, and one account can have as many listings as your settings allow.
If you want to remove the account completely
- Go to Users > All Users in wp-admin.
- Find the user and click Delete.
- WordPress asks what to do with anything the user owns. Choose to delete it or give it to another user.
- Once the user is deleted, the username and email address can be used to register again.
When you test the add listing form, use a separate test account and delete the user (not only the listing) when you have finished.