Does eBay actually hide listings at all?
Excellent question. It depends what you mean. Hard to say with any real certainty beyond eBay's admission that some listings are hidden for policy violations.
There are over a billion current listings on eBay, so I suppose one could say that any time your results show you less than that number, eBay is "hiding" some results based on the search criteria. But typically the folks that talk about eBay hiding listings are referring to listings that should appear in a given search, but do not specifically because eBay has chosen to hide that listing. Many of the instances of sellers complaining that their items do not appear in search fall into one of the reasons I listed above ("too soon"/"lowest first"/"wrong category"/"no shipping"). If those count as eBay hiding listings, then clearly eBay does hide listings.
If I were a seller whose listing did not appear in a "lowest first" sort, but the same listing appeared the same search using different sort orders, I would probably not be too happy with eBay hiding my listing unless the searcher used the "see more results" link. It is not entirely clear what criteria eBay uses to decide which lowest-first items appear by default and which do not, but it is likely based on some calculation that eBay makes about whether the item is sought by the user doing the searching -- rightly or wrongly. I think in some situations, the filtering is clearly going wrong, such as when a search for postcards that turns up millions of results in most sorts turns up only a single page of results when sorted lowest fist.
Setting aside those four reasons ("too soon"/"lowest first"/"wrong category"/"no shipping"), which users can get around by changing the way they are searching, and some technical errors which may or may not be gotten around at any given time, is eBay purposely choosing to hide listings for other reasons?
I have performed a lot of searches, and, aside from the odd seller going on vacation, I have not noticed items appearing and then disappearing from search and then appearing again, as one might expect if eBay were routinely picking and choosing which items to show or hide on a regular basis as some sellers seem to suspect. For the most part, items seem to appear when they are indexed, and disappear when they are sold or ended. But it is still possible that eBay is hiding things from me that I do not notice because they stay hidden and I never find them.
And aside from hiding listings that violate policy, which makes some sense to me, no one has really explained to me why eBay would want to hide valid search results from potential buyers that want to buy them, beyond a few hand-wavy explanations that eBay wants to encourage certain sellers or discourage others, or perform some sort of load-averaging, or make east coast sellers ship to the west coast and vice versa to maximize shipping fees (presumably sellers offering free shipping are exempt from this particular manipulation).
It seems to me that most of the complaints about listings that are "hidden" fall into one of my four basic categories, or else the policy violation exception, and eBay has a reason for hiding each of those types of listings, though some will disagree with those reasons. A few other rare odd cases can be explained by technical issues that are possibly the result of unintended interactions between two different "smart" features, or a strange asymmetric keyword substitution issue. Or the interaction of ad blockers and sponsored listings.
Humans are predisposed to find patterns, even in random data. And there are many common misunderstandings about how eBay's search really works -- misunderstandings that eBay does little to try to explain or dispel, unfortunately.
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