Practically everybody.
They are useful in some categories where there are only a few buyers, who are usually also re-sellers or speculators and don't want their customers or other collectors to know they are going after that particularly toothsome item.
BTW, the 5**6 IDs are not 'private bidders'. They are just ordinary bidders whose IDs have been masked for some years by eBay in an attempt to stop fake Second Chance Offers. The unintended consequence was to mask shill bidders a more pervasive and dangerous problem.
