Lots of folks will likely agree with you that there are problems with eBay's search. Oddly, no two of them can seem to agree on what those problems are exactly, when they began, or what should be done about them.
There is more than one way to search. Don't assume your buyers are necessarily searching the same way you are, that they are seeing the same results you are, or that they are expecting to. Different users using exactly the same search keywords can have vastly different ideas of what results eBay should be delivering. eBay cannot make everyone happy under those conditions.
There are probably some things you can do when searching to get results that are more in line with what you expect; likewise there are things that your buyers can do.
If you are using "Best Match" as your default sort order, try another sort order if you are seeing "too many" results. Likewise if you are using "Lowest First" and seeing "too few" results.
Choosing good keywords can help with a search. Grouping keywords with dashes so that the keywords appear in order with no intervening keywords can be a useful technique.
Using quotes or exclusions should bypass any keyword substitutions or corrections and provide only exact, literal matches of your search. Keep in mind that eBay considers any listing with your keywords in the title or item-specifics to be a potential match.
I see 740,000+ results when searching RPPC in the Postcard category when sorting by "Newly Listed". Using quotes only reduces that number to about 720,000+ results. Using "Best Match" sort order produces over a million results, with or without quotes.
Choosing the proper keywords, changing the category if necessary, and then using appropriate filters should allow users to find what they are looking for and avoid things they are not -- under most circumstances.
