Did your buyer open a case for item not as described? If they did, it's all automated, no human contract, so no point challenging it at that point. You Have to refund and do so within 3 days. If you don't ,then the buyer can escalate the case, eBay step in and force the refund.
If there is no money yet available in your Payment Summary eBay take the money from your back up source usually your checking account. Then, you can appeal the decision for the refund.
You are in the States and asking on UK board so those are our rules here, I believe the same in the States?
If you wish to appeal contact eBay Customer Services, but first you need to log out of this UK site, and log into your homesite, www.ebay.com and contact CS here:
Contact eBay Customer Services this way, through their help pages.
https://www.ebay.com/help/home
Lines are open between 5 a.m. and 9 p.m. Pacific Time
@jolosc14