For those buyers who have problems with sellers who will not ship to their PO Boxes, there is a workaround called PO Box Plus.
In essence, you need to sign up as it is an additional service, but what happens is that instead of checking out with "PO Box 123", it changes to "Suite 123", and should therefore note be an issue for sellers to post to.
I think there is a limit on the number of deliveries using this (maybe 50 a year?) service.
Worth checking out for some buyers perhaps?