If you purchase a standard Royal Mail letter label online it comes with a "tracking number". However, that cannot be used to track the item. Badly named, isn't it?
If you sent an item as a plain letter then you have no defence at all if the buyer says the item didn't arrive. You will have to refund. If you don't refund, eBay will instead (and will not give you the fees back).
Historically I got about 2% of such untracked items reported as not arriving, I'm pretty sure nearly all the buyers were lying - in nearly 10 years at my current address my regular postie has misdelivered 1 item (was recovered later), I've probably received nearly 10,000 Royal Mail items in that time - that's 0.01% not 2%. I think buyers are increasingly aware about untracked items and that they can easily get a refund for them. It is a shame that we live in a society with such pervasive low-level fraud but that's how it is.
If you want to be able to prove that a Royal Mail item has arrived then use their Tracked 48 service. It costs £2.70 (Large Letter), which is a lot more than 75p, but that is the cost of proper tracking and insurance. (Don't bother with Signed For, it costs same/more and has less reliable tracking). Otherwise, if you send as a letter then you have to rely on the buyer's honesty.
Royal Mail Large Letters (£1.55 for 2nd class) should come with delivery confirmation, which counts as Tracking, but in my experience those only get scanned around 80-85% of the time, so it's not a good enough service if you must have Tracking.
