If you read the fine print in just about any TOS or contract, the account is property of the system owner and the user is allowed access to the system at the discretion of the system owner. Account termination usually can occur without warning, justification or appeal. The account (and often any data within) remains the property of the system owner. In this case, eBay.
If I were eBay, I'd be investigating the application of "accessing a system without permission" as it relates to the private investigation company.