On ebay, can I create a private blacklist?

You can create a BLOCK BIDDERS list where they are unable to bid on your sales.
Managing bidders

blocked bidders list
It’s totally private and they just get a polite notice saying they are unable to bid.
easy peasy – lemon squeezy

Scotsbloke nailed it. It’s very easy to create a list of blocked bidders. Personally, I block anyone who seems like they’re going to be difficult, let alone those who don’t pay, etc.

You can also change your profile to disallow certain categories of buyers – e.g., those who have had an unpaid item strike, or who have an overall negative feedback score this last one, of course, is meaningless now since Feebay eliminated negs for buyers