Although this advice will more than likely feel as though it has come too late to be of any use in the matter which actually prompted you to create this thread, it would be well worth remembering for the future in order to avoid a recurrence of the same problem.
Whenever you post your items out, make sure that you always used a postal service that provides tracking and upload the tracking number at the earliest available opportunity after you have posted the item out. By adding the tracking number manually against the purchased item eBay will automatically mark the item up on the buyer's eBay account as having been despatched, and if you get any messages from the buyer alleging that the item has not been received but the tracking number proves that it has then quote the tracking number in your reply to the buyer, as if he/she realises that you can prove that the item has been delivered it may well put him/her off of opening a false Item Not Received case against you.
In the event that you find yourself in the situation whereby the item has been delivered and the tracking number proves this but the buyer is saying otherwise make sure that you upload the tracking details to the eBay Item Not Received case. That way when eBay review the details of the case and put a trace on the tracking number they will close the case in your favour when they see that the item has indeed been delivered to the address that the buyer supplied when he/she paid for the item.
In addition to the above, if you find yourself in a situation where you actually end up winning an Item Not Recieved case but the buyer has left you negative feedback in the meantime get onto eBay Customer Services straightaway, stating that given the fact that they decided the case in your favour the buyer's negative feedback should not be allowed to remain. Under those circumstances eBay would remove the buyer's negative feedback, as well as the associated defect, in which case your account would not have been affected by anything that the buyer has done or said.
Finally, if you have not already done so by the time you read this response, add the buyer's User ID to your Blocked Bidders List so that he/she cannot purchase anything else from you in the future and cause you any further aggravation. It would also be worthwhile having your Selling Preferences set up so that the little box is ticked next to the sentence that says "Don't Allow Blocked Buyers To Contact Me." You only have to activate that option once and it will remain active forever unless you change your mind for some reason and decide to deactivate it. Once that option is active, however, nobody who has been added to your Blocked Bidders List will be able to contact you, although given that you have recently been in a transaction with the buyer who you lost the case against the blocks would take slightly longer to kick in than would otherwise be the case.
