I have a feeling that I may have 'solved' the issue.

I NEVER leave FB for anyone till I've posted/couriered.  Order is post, then leave FB, been doing it that way for years.

According to live chat I've already left FB, so the assumption is that eBay left my standard one on my behalf.  And somehow that knocked out the 'mark as sent' bit on the list of options.

It's the only thing that kinda makes sense.

I'll be interested to see what happens on my next sale - hopefully that will be sooner rather than later!!