ebay nixed private feedback for sellers a while back. It's probably a good thing - if something truly needs to be private, private
auctions can be used, but buyers really do need to see a seller's feedback to make an informed decision on whether to buy from them or not.
After doing a bit more research, it seems this person in particular has made thousands of bids in the last 90 days. I didn't check every single one, but the ones I did peek at were always just one bid per auction, in the lower dollar levels (as compared to where the auction ended up). So they are a bargain hunter, perhaps, bidding on many and ending up with very few. I probably won't end up with them as a buyer unless nobody else bids.
General purpose research shows a majority of people think bidders with private feedback have something to hide, with only a few people chiming in with plausible reasons why someone might have it. I think you, CollectorCorner, actually made the best argument out of everyone I've read. I can certainly understand someone with the mindset of, "It's nobody's business what I buy." Privacy, or lack thereof, online being what it is, it makes sense for some people to get it where they can.
As a seller, personally, I try to get to know my buyers a bit, even before they win. I look through feedback of bidders sometimes, and
always of people who make offers. I like to see a potential issue before it arrives so I can try to avoid it. Taking away that tiny bit of transparency is disconcerting.. but sometimes you just have to take a chance. If you cover your bases in every other way, there's a high chance of it being a positive experience.
jdmern, buyers with 0 or very few feedbacks make me nervous, too. I've sold to them, but made sure to cross my T's and dot my I's, and kept my fingers crossed as well.
basebal21, you seem more relaxed than most sellers

Yes, many feedbacks are left automatically by big sellers, but there is other information to be learned in their feedback that could be enlightening. It takes time, of course, and not everyone finds it worth the trouble.
Thanks, all, for the comments.