Look. I'll tell you to be one, too in this case. 
Don't refund her partial payment, send her an 
email saying, "Your part of the transaction was 
only 50%, mine will only be %50, makes 100%. 
Sounds right, ne?" and slap her a 
baaaaaaaaaaaaaaaaaaaaaaaad feedback.
eBay allows a treshold of screw-ups most of the 
time. So permit yourself the buttkick on her, we 
all need to be mean at times >)

Ore-sama