In answer to your second question, we (other members) don't handle it
You (the seller) does by cancelling with the reason 'did not pay' (or similar wording)
The 'buyer' will get a strike against their account from eBay
And then you add them to your blocked buyers list
I don't know if you can set up immediate payment when your listings all have 'or best offer' on them, but I have never done a or best offer so I will leave that to those who do
