Which listing and how are you searching?
There are many reasons why a listing might not appear in a given search.
Too soon -- items can take 24 hours to be indexed and appear in a search; longer if your revise the item before it appears.
Sorting by "price plus shipping: lowest first" -- when a lowest first sort is performed, you will often see a message at the top of the results: "We removed some search results to show you the most relevant listings. View all results" -- if you want to see all the results, you need to either try another sort, or click on that "View all results" link. Otherwise you are only seeing a subset of the results of that search.
Wrong Category -- if a site-wide search using a particular keyword returns many items, eBay will often direct that search automatically into one particular category. Sellers should take this into account when deciding which categories to list their items in. If possible, try searching the way a potential customer might, and see where the search ends up.
No Shipping -- if an item does not have a shipping method to a specified location, buyers in that location will not be able to see that item. This is often a problem for export sellers that do not ship to their own location.
On Vacation -- if you use the "Time Away" feature, your items may be hidden from search. After ending a vacation it may take a day or two for your listings to be re-indexed and appear again.
In addition there are some known location-specific issues that affect users in certain locations like Hawaii and Alaska that may prevent them from seeing local items.
