Australian PayPal accounts can be verified or unverified, and PP can also be used as a guest - my emails from guest PP accounts don't have eBay details in the subject line of the email, the format is very different and it says "You've received new funds" at the top.
On eBay, the status of the buyer's PayPal account doesn't matter too much as you can qualify for seller protection through all the normal means. Off-eBay, however, you can't get seller protection if the buyer uses PayPal as a guest (at least, they were the T&Cs for PayPal as of last reading, which was admittedly a while ago).
eBay also has the option to block members that haven't verified their eBay account, which is a bit different to PP verification. I do block members who aren't phone verified, though my items are mostly low-value, so not typically the kind of high-risk items where a seller would find this block most attractive. The reason I first implemented the block was because I was specifically attempting to prevent a problematic member from continuing to make new accounts to try and purchase from me (new accounts almost always are unverified with eBay). I also set it up so the block applies to all members, though, not just the ones with FB below a certain level, because this member was using multiple accounts (new and old) to target me, and I just wanted every block in place I could have.
I've had it in place for a couple of years now, and I commonly have around a dozen pages in my activity log of members attempting to purchase unsuccessfully due to this (I can't remember how many it displays per page, but I think it's either 25 or 50? so I guess that means it's either 250 or 500 attempts every 3 months (most members try several times over a couple of days, though, sometimes over a couple of weeks).
The reason I kept the block in place after the initial troubles were taken care of, is because I noticed the number of problematic transactions also decreased once I set it up - I know it may have been coincidental, and that not everyone who doesn't verify their account is going to be a problem; I have also exempted plenty of people from the block, or advised how to verify an account when people contacted me about it, but it seems to work for me so I kept it.
The eBay block can be overridden if the member goes through phone verification, links their eBay account with a verified PayPal account, or puts a valid credit card on file with eBay - all things that add a little extra layer of identity confirmation, which makes me just that little bit more comfortable.
