Outstanding Sellers Cost for Refunded Item I was Unaware Of

I'm a college student trying to sell some personal belongings to help pay for school and living. I listed a tuba I no longer play on eBay in hopes that it would help significantly. After the holidays, I noticed I had a very substantial negative balance on my account. Over $700! After some further investigation, I had discovered that somebody had purchased the tuba during the holidays and I had no idea! All of the email notifications eBay sent were flagged and automatically sorted into my spam folder where I did not see any of them. Obviously the buyer submitted a refund request because I was unresponsive and because I wasn't notified of that either, eBay had to step in to close the case. I was also sick during the holidays so I wasn't making a point to log into my account to check. Now I'm stuck with such a large sellers fee because the cost of the item was high and I cannot afford to pay it. I tried submitting an appeal to no avail and I don't know what to do!!! Has anybody else experienced an issue like this? Is there any other way that I can get the fee disputed or maybe even just reduced so I don't have to pay so much? Or am I just totally and utterly screwed?