I need some eBay/USPS advice

In My Humble Opinion
1

TL: DR version:
USPS might have opened the media mail boxes I shipped and switched their contents around.

Full story:
Almost a month ago I sold a lot of my sheet music collection on eBay. There were several folks that bought a lot of books at once and so their shipments went into boxes. Off they were shipped. Then I got a couple emails from the buyers that they hadn’t received their books yet. They were all from the same shipping date and they were all boxes (judging by the weights). Then one buyer got books in a damaged box that weren’t his, but books of two different buyers. He then opened up a case on every single one accusing me of trying to pull one over on him. I then showed him all of the books that I sold and he’s still dubious.

Now, I had 4 eyes when I was packing these. I would hand the pile of books to my SO, we would review each set before putting the books in the box to make sure that we had the right amount of books and the right buyer’s address. Everything was double checked.

So, this has affected at least three buyers. One of which I’ve heard from who received the incorrect books and is shipping them back to me (I’m reimbursing him for the shipping). One who hasn’t received any books at all. And a third I just discovered today, I just emailed him to see if he got the books and which ones he received.

On top of all of this, I was dumb and didn’t opt for tracking or insurance. USPS did warn me that since it was being shipped Media Mail that packages would be opened. I’ve got a feeling that the books might have gotten switched around during that or if some of the boxes opened they just put them back in willy-nilly.

So any advice on how to handle this?

2

Media mail, when you can use it, is way cheaper than any other option, assuming your buyers can handle the delay. For your situation, you refund your money to anybody who didn’t get their books, resend priority mail misdelivered books, and thank the buyer who shipped back the mis-shipped books. I don’t think you’ll get much help with media mail problems, because of the cost.
You should almost never get shipping insurance- it almost never covers anything but a total non-delivery where their tracking admits there was a non-delivery (as opposed to being delivered and stolen or delivered to the wrong address). IMO you should treat all buyer issues as a cost of doing business and build it into your overall prices; if buyer issues cause you to lose money overall, then you just can’t profitably sell whatever you are trying to sell.