There is most certainly something you can do about this - I can not see neg on your account, so assume the buyer is 'threatening neg' !

You have offered to pick up at your cost, refund on return, no questions asked, they have demanded monetary compensation that they are NOT entitled to.

If you do receive the neg they are threatening contact eBay as soon as you can, keep all correspondence between the buyer and yourself, this is blatant feedback extortion.

Speak to a supervisor about this, do not accept the blah blah from the phone drone that answes your call.