It is very unusual for a scammer to leave feedback, I suspect it may for once be you who did something wrong with that buyer. I am an experienced member who has been buying and selling on ebay for over a decade and even I would be cautious about selling an iphone on ebay or any other on line site. With no feedback for selling and now sitting at minus one I doubt if any legitimate buyer would go near your listing anyway.

I would think the only way to sell would be using a very popular social media site where the buyer comes and collects and hands over cash.