Good question, for the first time I have exactly the same thing!
The Ricardo page is here:
https://help.ricardo.ch/hc/de/articl...nicht-bezahlt-
They say you can cancel within 7 to 60 days to get a refund from them of the listing fees, it isn't a suggestion on how to handle the buyer. They then give the buyer a negative rating.
I've emailed and texted. Will text again on Saturday in case they're on holiday, and try calling on Monday.
If I don't get a response within 14 days I will cancel the sale and relist them.