eBay/Royal Mail problem?

[The Shadow™ ®]

this seems to be a screw up on your end , you should always get a tracking number for packages and insurance that covers claimed value ( even if its more expensive it is worth the cost ) so you can prove if the buyer received it and if it got lost can claim on insurance

if the post office lost it or it gets delivered late and the buyer is dishonest and doesn't return it then you are out the cost of shipping less any claim you can make , and the cost of the item
this happens regularly when you ship parcels , experienced sellers will always pay the extra cost to have tracked shipping insure for true value .
I hope next time you sell online you remember this and use it as a hard lesson
good luck in getting it sorted , hopefully the post office will find the package and it gets returned ( unlikely but it's been known to happen )