I would have expected eBay to give you advice, but keeping your account secure is your responsibility. Have you read eBay's guidance on hacked accounts: https://www.ebay.co.uk/help/account/protecting-account/get-help-hacked-account?id=4196&st=12&pos=1&q...
Assuming that you have control of your account again I assume that eBay has guided you to change the password?
The concern is how whoever gained access to your eBay account was also apparently able to pay for this £300 purchase. Is your account linked to a PayPal account? If so, how were they able to access this account too? Surely you didn't use the same password for both? Did you have the "one-touch faster payment" option enabled? Even if you did, this is only supposed to be enabled on a connected device on which you had previously logged in. So an external hacker should not have been able to access it.
You haven't told us whether payment was taken from your PayPal account, or from some other source of funding? The point is that eBay is not involved in making payments. You need to be talking to PayPal, your credit card issuer or bank account, whichever was used without your knowledge to pay for this item.
Saying that you "just want a refund" is an over-simplification. Whether you get refunded may depend upon whether you took sufficient care to prevent your account from being compromised. Goof luck, I hope you can get your money back.
