As some users may already be painfully aware,
ebay has been touting "improved search". The results in the Stamps category could deter buyers and devastate sellers. Try the following search:
1. open the US
ebay.com in a browser.
2. Navigate to the Stamps category. Has anyone noticed that the drop down list is too short and extra clicks are required to expose "Stamps".
3. Search for "500". (The quotes are not needed in the search. Unfortunately,
ebay believes that key words will work, but doesn't understand that stamp key words are usually catalog numbers.
4. In the left panel, navigate to "United States" under "Stamps".
5. Now try to navigate to "1901-Now: Used".

At this point, a buyer looking for a used US Scott 500, may find some other way to reach the category, give up or go to some other site. While it may be possible other posters to offer "advice" on how to search
ebay or what other site to use, such help will not be seen by the most potential stamp buyers viewing
ebay. Also, notice how poorly search for "500" as a Scott number works on
ebay.
The left panel has several shortcomings. First, is including irrelevant categories. How does "
ebay Motors" or "Business & Industrial" relate to stamps. Why do they appear in the left panel at all? I know that
ebay loves out of category searches, but I don't believe that buyers share the love.
The second shortcoming is that the list of subcategories under "United States" is incomplete. There is no way to expand the subcategory list in the same manner as the "Show More" will expand the irrelevant list of other non-stamp categories.
The Search at the top of the page allows navigation to categories above, but not to ones at the same level or below. No help here.
It is sometimes, but not always possible, to open a specific listing and navigate to "United States Stamps" category where more subcategories appear without irrelevant non-stamp categories. By this time, many buyers will have dropped out.

Also, the check boxes below the useless Guaranteed Delivery radio buttons may not be helpful. It is not clear how many sellers manage to use matching item specifics with continually morphing descriptions, some considered illiterate by philatelists, and unsupported in third party applications.
Using the check boxes may be an exercise in futility if only a small fraction of potential matches are exposed.
The only truly useful check boxes are "Sold Listings" or "Completed Listings".
Finally, a couple of times, searches have morphed into "Virtual Categories" showing left panel choices not fully related to the original category because some filter was incorporated limiting search to something like "collectible" items. If that is the case, who decides? Why does search have to hide? Does
ebay have something to hide?