I am assuming that the item was the thermomix TM5 & that what you mean is you demonstrated that it was working before the buyer took it away with them.

When a buyer claims item is damaged, the seller has to accept a return and then issue a full refund.

Given the cost of the item, I hope you got it back before you had to refund.

You can get through to a real ebay person but you have to persist. Go through help pages to chat and keep going till you get a call back option or can chat to a rep and not just the Bot.

If you have already got the thermie back and it is not working properly or damaged, then it could have been caused by the buyer but you have no real way to prove it. It is an intricate piece of machinery with dozens of things that can go wrong.

Others may have some advice on what you can do in this particular case but my advice is for the future.

Pick up items-marketplace. Not ebay.

Items like TM5-marketplace or even gumtree, not ebay.

Whenever you have anything with lots of moving parts for sale or anything electronic, the danger is there that a buyer will make a claim on ebay. Sometimes they just don't know how to use something, sometimes they break parts, sometimes they are a bit scammy. 

But what I am seeing is you have a feedback rating of zero, so you look fairly new to ebay.  Selling high priced items like this is not the way to start, too much risk.

You asked others have had something similar happen. The answer for me is yes, not for anything in this league but years ago sold an item, buyer claimed faults, could not even show a decent photo. We knew there were no faults but we also knew ebay usually sides with a buyer so we offered a partial refund as the most cost effective way to get out of it, rather than paying for a return and refunding (it was an overseas sale). The buyer accepted.

That won't work here as the case has been settled, by the sounds of it.

What is wrong with the thermie, by the way? You may be able to reset it. There are some thermomix groups on facebook that can help if you have a problem, before you have to send it away etc