I doubt that the problem lies with the printing of the labels, as many people use without problems.
Have you checked your information loaded onto ebay, particularly addresses. Did any kf the items have tracking, if so what does traccking sayS.
There are 5 address boxes all for different purposes, and I think you need to check that delivery address is correct.
Even though undelivered, I would have though some of the 8 items would have been returned to sellers. Have you asked, to see why the items were returned, as that might help solve the problem.
It might also be worth advising your local delivery office of the problem, as unless they are advised, they cannot investigate.