You can narrow your search with boolean search string expressions. eBay
does recognize some operators for example if do a search with in US Stamp for:494
You'll get all kinds of junk. So first get a little more specific with 494 Washington
or SC 494 Washington
or add Scott, 3 cents, etc... Just be aware that the more specific you are, sellers are inconsistent and you may miss some ads.
If you put in: 494 -cover -covers -panel
It will remove many of the stamps on cover that come up. You can use multiples to tailor your search. I usual start out using the simple search and start adding -term to remove the junk. As you see more items you don't want to see, keep adding a -term to remove them
Make sure you put a space between each and no space between the minus sign and term. Just as shown above.
If you click on advanced search by the search button, you'll find some options there to tailor your search too.