If the postage options that eBay offer do not suit you and you find it awkward to purchase and print labels direct from Royal Mail's website then perhaps the following workaround option would work for you:
First of all, decide on how much you want for the item, then add on a bit more to compensate for the amount that eBay will take off in fees, as well as adding the postage costs onto the asking price. Once you have done that list the item with free postage. That way the buyer cannot complain about the postage costs as the question "How reasonable were the postage costs?" will not appear on the buyer's feedback page when he/she comes to leave feedback for you. In addition to that you'll also get an automatic five stars for postage costs from eBay, just for listing the item with free postage.
When it comes to posting the item after the buyer has made payment go down to the Post Office and purchase the postage label in person. All you need to do then is to stick the postage label on the parcel and post it to the buyer. So long as you use a tracked postage method and upload the tracking number as soon as possible after posting the item the buyer and eBay will both be aware that the item has been despatched, and once the buyer receives and signs for the item the presence of a signature proving delivery of the item should help to bolster your case against the buyer in the event that he/she makes false Item Not Received allegations against you in the hope that eBay will refund all of the money paid. Most buyers are decent, honest people but it certainly pays to protect yourself against the minority who are just out to screw you over.
