I think my original source was this communication from Crowdcube that @iansilversides posted last year:
It’s been quoted by a few others since then but without any new primary evidence (or so my forum search tells me).
I’ve seen lots of requests for shares but only two or three offers I’d suggest were serious so my sense is that this is quite rare if it happens at all. Purely a personal view, though, since no one ever comes back and confirms a sale.