Their search engine employs a set of rules. One, it does what's called "fuzzing" (look up 'Levenshtein distance'), where if it doesn't find an exact hit it "fuzzes" anything even remotely related. Another thing the search does is search mnemonics, so if you search "HP computer" and there's a Harry Potter computer mouse, that will appear. As people mentioned, best way is a literal search enclosed in quotes.
That said, even if the item's title is say "Cool Lamp Shade" and you search that in quotes it often still won't appear because their search is wonky and has been broken for years. Stuff doesn't even appear in my Seller Hub when I type it in literally. You might try the "picclick" website as well, though it's bound to the same rules. Last ditch effort, try google. You can type this into google: site:ebay.com "cool lamp shade" and it will return listings that match (but again, not perfect).
