Thank you for all the recent comments and advice.
I'll put this down to experience. Hopefully others will learn from my mistake too.
I opened a case with ebay the seller accused me of making 'threats'. I have maintained very polite language with her the entire time. I was perhaps a little direct in saying that I felt there were two outcomes: (1) return or (2) partial refund - but I don't think this is a threat. She then said "hopefully you'll learn a lesson and in future ask whether there is fungus before you bid like everyone else".
This experience is leaving a very bad taste in my mouth so I decided to end it. I left her the following negative feedback:
"Failed to describe large amount of fungus in lens. Refused return. Aggressive."
I have lodged a complaint with ebay and will be interested to see if her account is suspended.
Hopefully other ebayers will not get stung by this dodgy vendor (I think she is genuinely confused).