Thanks for advising, especially information contained within the second paragraph. I’ll keep that in mind if another situation like this presents itself, and fingers crossed it doesn’t.
As I suspected the seller turned out out to be a nasty piece of work. I decided not to revise my feedback after they left me a ranting false positive not even 12 hours after sending me the first hostile message which had basically ordered me to revise my feedback. Plus four more borderline abusive messages following.
EBay removed the false positive and told me that despite 100% positive feedback the seller had a bad record
A really unpleasant waste of my time - and disappointing too - but the seller did get to borrow my money for 3 1/2 weeks, so it worked for them for a while.
Looking back I doubt the seller ever had the bag I bought in their possession.