One problem with doing that is that sellers frequently don't check off the various items, so it excludes them if they're left blank. I don't want to miss an item that way. So I try to find unique words in the ads I want to exclude and add them to the "-" part of my search string.
Often, if you click on "Isuzu" under brand, you'll still get a ton of garbage...because the seller wants you to still find their listing, even if it's not "Isuzu" brand so they've checked that box...so I still try to find enough unique words to exclude, to pare it down to a manageable size number of listings.