The ONLY way to prevent a bad transaction from becoming a nightmare is this. If you have made a purchase and you are not happy, yet the seller refuses to cooperate in a solution, then leave your feedback. But BEFORE you do, please do the following! Block that seller from coming into your store so they cannot purchase from you. Make sure you use their user ID and not the store name, otherwise it will not work. Add them to your blocked bidder/buyer list. They will now be prevented from purchasing anything from you and thus, cannot leave a negative feedback. Although eBay does not allow a seller to leave negative feedback, they do not prevent that seller from coming into your store to purchase from you after the fact. After they have purchased from you, they can then also leave you a negative feedback and low DSR rating which can greatly effect your selling on eBay. I had this happen to me and do not want it to happen to you.
So remember sellers! Protect yourself! Block those unprofessional sellers before you leave feedback to prevent this from happening to you.