If you go on the 'personal details' section of your ticket account there's a box top right that says "Bought tickets but cannot use them? Click here to sell them" Click that and follow the instructions - think they take a percentage though, the cheeky buggers.
Season tickets get a full refund don't they? Surely should be the same for members?
EDIT
It's a full refund
https://www.liverpoolfc.com/tickets/ticket-exchange Official Members
The Ticket Exchange is now also available to Official Members who will receive 100% of the price you paid for your seat if your ticket sells on the Exchange.
Click here for a step-by-step guide.
Flexible ways for you to receive your refund if your seat sells
1. Choose for your refund to be added to your new Ticket Cash account which can be used against future online ticket purchases.
2. Choose for your refund to be transferred directly to your bank account.
Refunds will appear within your Ticket Cash account or Bank account within 10 working days after a game has taken place.