as a new seller return help

[sara6657.wrtsk]

a buyer is demanding that they return a item that was in mint condtion to be returned, anyway i have 

agreed and refunded some postage 

now three days in its nonstop insults and threatening me with bad feedback my seller account etc 

if i dont pay double postage, i get a feeling they want me to just say look just keep it

as its a low won item

i sold it on a uk site i tried to get ebays help uk site but was told buyer was on the us site

but i shipped it in the uk

 can i stop all this by getting ebay just to take over

get this buyer just to send it back

 never ending messages full of insults when everytime am like 

look please just return the item i WILL refund both postage and item plus i already 

given you money for postage 

please help