In my experience, the buyer has the ability to redirect the package before it has been returned if the courier is given an address they can't deliver to - not sure if that's the case with your courier, but if so, the buyer had two options (as in, provide a deliverable address, or redirect it to a deliverable address - and addresses are the responsibility of the buyer).

That being said, I'm in two minds as your listings don't seem to indicate that you use a courier, except for the express service. Sometimes I buy from a seller and choose standard delivery via Australia Post, but they decide to upgrade my package to express via DHL or UPS, which can be really annoying because I like to use my PO box for as many parcels as possible to avoid having to wait at home for deliveries, so then I have to make different arrangements - I do this as soon as soon as it is possible, though, so the package is never returned to the seller. 

I guess wht I'm saying is, if I were the seller, if the buyer knew that a courier was being used, they should bear responsibility for providing an address the package couldn't be delivered to, if I hadn't made that clear to them, I would accept the responsibility.