So how do you do that? I would definitely take advantage of that feature!
It's definitely buried. I originally asked how it was done in my post because I couldn't figure it out, then I suddenly found it!
It's also quirky to update a search. Do an advanced search, add the terms and/or excluded sellers, etc., but before you do the search again you need to check "Save this search to my Ebay" near the top (awkward when you're updating excluded sellers). A popup appears, and if you want to replace an existing search you have to click the "replace search" link in that popup.
I do UI design work as part of my profession and their search UI is hideous.
I've been adding excluded search terms like "Mask Neg" and "tee shirt" to take care of phrases I don't want to see. Also "Flashback silver" and "Flashback 2 silver". Fun!