Nearly the same as page view counters elsewhere on the web, the Hit Count on eBay is how many people have viewed the item's listing page. Many sellers include eBay's standard, or their own 3rd party, counter in their listing(s) to show how many people have "passed by" the page. While there can be some variation in how the internal tracking of such counters distills down to a final figure, these counters aim to approximate how many times an item page is viewed, usually also striving to ignore repeated views by the same person. This metric might be analogous, in a brick-and-mortar retail store, to how many people pass in front of the store's window and turn their heads to peek through at a certain product on display.
Moving a step closer along the path of visitor interaction we have the Watch Count, as it's often termed. This is how many eBay users have actively clicked a "Watch This Item" link on the listing page to express some kind of interest in it, whether as a bidder, potential bidder, fellow seller, or other onlooker. This affirmative action of the user intentionally clicking the "Watch This Item" link/button, and being logged in to eBay at the same time, makes the Watch Count an excellent "popularity vote", much like other user flagging technologies employed by other websites. Taking our physical storefront metaphor a step further, the count of how many Watchers an item has amassed is akin to one of the window shoppers choosing to enter the store, walk over to the product on the shelf, and inspect it further.
Like in many other online and offline auction venues, the Bid Count is of course how many bids an eBay auction listing has accumulated. To place a bid, a buyer must be logged in to eBay, enter a bid amount, click a couple links successively, and possibly have placed a credit card on file. Several small hurdles must thus be traversed before the Bid Count increments by 1. In our metaphor, this might be similar to how many people have not just examined a certain product on the shelf, but have taken it up to the cashier with a bona fide intention to purchase it.
Each of these counts can be a measure of item popularity in its own right. Let's explore some of the differences, pros and cons:
The Hit Count is rather broad, measuring casual item interest "from afar", but it includes counts from non-eBayers as well. Many eBay sellers include a standard such counter in their listings to boast page/visitor views, suggesting a certain degree of magnetism that pulled searchers to the item from various search results pages, on eBay and off. You'll find that often items on eBay Pulse and charity auctions can display tens of thousands of hits as a result of their listing promotions. However, as such a broad measurement, the hit counters are often thought to only marginally add to the marketing value of the listing, and probably only detract in a minority of circumstances (such as when a listing is coming to a close and has only accumulated a few page views).
Moving on, we note that for the Watch Count of an item to increment, it requires an actual click from a registered eBay user. As such it makes a great "popularity yardstick", a kind of vote from the eBay community. In contrast, the Hit Count accumulates regardless of login status, and one certainly doesn't need to click on anything to make it move up a notch. The Watch Count intermeshes interest from both buyers and sellers (and others), so this is often thought of as a negative by sellers seeking to acutely gauge shopper interest.
Finally, Bid Count is of course naturally a prime choice for measuring direct buyer interest in an item since it requires registered shoppers to make legally binding contracts (bids) and pledge actual money. How many bids build up on an item almost always demonstrates a bona fide level of interest by shoppers. One thing to keep in mind, however, when eyeing this metric in comparison to others is that bidding often doesn't accrue much until the end of the listing, whereas Hit Count and Watch Count accumulate throughout the entire lifespan of the listing. Moreover, using Bid Count as a popularity metric misses the many potential bidders who have some solid interest in the item but abstain from bidding for various reasons. (And if you're using eBay's Classified Ad format for your listing, it can't be bid on at all.) That being said however, in the expansive terrain of auction-style listings, the Bid Count is often considered the pinnacle of yardsticks for displaying item popularity.
It's interesting to note that we can see a progressive degree of "engagement" that each of these counts reflects. The Hit Count seems to capture the broadest and most cursory level of interaction with an eBay item, while at the other end of the spectrum is the Bid Count which historically is the most blatant and visible indication of item popularity, at least by shoppers. In between these extremes, the Watch Count harbors a mixture of both plusses and minuses from the other 2 counts, capturing both the genuine yet generic interest of any registered eBay user, yet falling short of the kind of commitment that a bid entails.
With rare exception, the Watch Count of eBay items isn't available for comparative activities, as it's normally a private figure available only to the item's seller. (Specialized software tools, such as here on WatchCount.com, and other opportunities sometimes surface to make Watch Count data available to the public.) Similarly, the Hit Count of an item can be hidden from public view, but often sellers prefer to leave it on display. And of course, on auction-style listings the Bid Count is certainly prominent and available by all for inspection and consideration.
Naturally, when evaluating a potential eBay purchase and utilizing these counts for comparison, it's important to take item/category context into consideration and not end up comparing "apples and oranges". For example, it's common for Watch Counts for cars and other eBay Motors items to reach past the 300+ mark, while the Watch Count on Pez dispensers typically hovers within the 1-2 dozen range. Likewise, Bid Counts tend to have more "meaning" for comparative purposes only during the last minutes (or after) of a listing, so side-by-side item comparisons should take this into account.