Can a rating be created for an incomplete transaction?

This depends on the circumstances of that transaction. 

If a buyer has paid money to the seller (whether complete or partial) and the seller backs out of the deal or otherwise breaches the contract between buyer and seller, then the buyer MAY leave a rating sharing that experience with others. Other potential customers have a right to know the seller might not be reliable to complete a transaction. 

If the buyer has paid money to the seller (whether complete or partial) and the buyer backs out of the deal, no matter how valid or sincere their reason for backing out might be, they may NOT leave a rating for that transaction. If a buyer has become suspicious of a seller, but the seller has not actually breached any part of the contract, then the seller should be given an opportunity to follow through and complete it before any judgement (rating) is rendered.