Answer to question (a) is nothing.
Interest cannot be earnt on what is called Electronic Money.
Nothing you can do to make buyer collect, in the event of a not received claim, attempted delivery is good enough for seller to win.
It is ebay’s site, they make the rules, you have the choice to not sell on ebay.
You do realise that even when you have the money, buyer using the 30 day money back guarantee, or a charge back through payment provider, you can still have to refund up to 6 months into the future.