eBay's search has some quirks to it, so where you end up depends on what route you take. If you start with a general search, then narrow down the categories, there are some extra filters that come into play. For phones, there are filters for individual model numbers and operating systems and memory and screen size and even CDMA vs GSM. Once you start using those filters, that should eliminate most of the extraneous accessories and cases without having to resort to exclusion terms. I think using exclusion terms interferes with the ability of eBay's search to figure out what you are looking for and present you with the appropriate filters.
Just today I noticed that my under the "Recently Viewed" section on the main page, eBay is pushing a series of pocket squares and handerchiefs, "Because you recently searched: -tie". Uh, thanks.
In searching, the trick is to strike a balance between false positives (the search finds things you aren't interested in) and false negatives (the search eliminates things that you are interested in). For really rare items, it's better to wade through some false positives to find a gem, than it is to miss it completely with too narrow a search. For more common items, as long as there are good options to choose from, I don't necessarily mind if I'm not seeing every possible match.
