The £500 + synthesiser? Firstly selling to a low or high feedback showing buyer, does a postage cost of £3.94 cover the insurance value of the item? I don't think so.
First I would find out that cover, and even pay the difference , cover the postage loss yourself if you have to.
There's always a risk in selling a high value item, no matter who buys it, but you've taken that risk by listing and now successfully selling the item on eBay. Post only to the name and address eBay provide on the delivery details, post to not other.
Have they paid? We all started with a zero feedback , some may even join eBay to buy that one item.
@abstracteye65