You are responsible for your key presses on eBay, whoever makes them. I hope you keep little fingers well away when you're banking online!
If you're saying that you were given the option of accepting a partial refund, and accepted it, then you've accepted it. EBay doesn't allow cases to be reopened.
If you paid by Paypal I suppose you coud let the eBay case lapse, and then open another under PayPal's longer, 180 day buyer protection policy.
