Depending on the value of the item, is it worth even posting back? Ie: if the item cost $10 and you're posting it at your cost, it doesn't sound worth it to even bother. Since the seller has offered a partial refund, it might be worth it to accept that, again, depending on cost of item and cost of having to post it?

Once the tracking shows the item as being delivered, it's possible that a full refund is automatically instigated anyway. After all, the seller can't get the item back and keep all your money. However in this case, you're the one out of pocket on postage.

In future, if an item is defective AND the seller wants it back (for whatever reason - perhaps repair), only then will you send it on the sellers dime. Otherwise get a refund and bin the item.

Most often if an item is defective, the seller will not want it and will need to refund you in full, so there will be no postage hassle, providing you open the correct case.