If they do neg you call ebay as sometimes they will remove it depending on the circumstances.

Tell them you can get written proof from a third party to advise the buyer had a change of mind as the item is in perfect working order as per the listing.

IF it does happen reply stating refund issued incl return post for working item.

This will show you are willing to help.

If a buyer opens a dispute again let them go ahead with it as paypal will tell them to return via registered post at their expense.

If on receipt of the item there is a fault then you can if you want refund the return postage at your discretion.