I have been able to list several fixed price items in sandbox, but my AddFixedPriceItem calls are now failing with ErrorCode10067.
One of the reasons why your calls are failing is because you do not meet the minimum feedback score requirements. It is possible that your sandbox feedback score was reset due to somebody posting feedback for your account. You can fix your feedback score by executing the ValidateTestUserRegistration call in Sandbox. Your listings should go through fine once you fix your feedback score.