I’ve had to explain to quite a few people that the current situation means that post is taking longer than usual and that ‘delayed’ is not the same as ‘lost’.
I’ve had to do the same with one or two things I’ve sold over the last few weeks. Most buyers have been fine about things taking a few extra days but one was emailing me every 2-3 hours after what he’d bought (a Scalextric car) didn’t arrive the next day after posting with Royal Mail. I gave him the tracking number so he could see it was on it’s way and the big message on their website stating there would be delays but he continued to email me every few hours. It turned up 6 days after posting. Wasn’t a kid either, some guy who worked for a big tech company going by his email address.
Just tell your buyer you are relying on Royal Mail declaring it lost first, understandable with current delays. Until then it’s still in transit.