It would be fairly trivial to modify the Ranged Price Bookmarklet to filter out such results (when clicked would make watched results partially transparent, remove completely and substitute a note, etc) *IF* there were something to grab onto in the search results that separated watched from not watched items.
Unfortunately in Firefox 31 I see absolutely nothing visually in the search results, or hidden in the search result page source that indicates any particular result is watched (checked 2 on my test watchlist).
Nothing to latch onto to differentiate results means no filtering possible by those methods.
Might be possible for a script to access the watchlist, and match those entries against search results for filtering, but that realistically isn't going to happen (I have no interest in that functionality, and nobody would use it anyway because it is a GreaseMonkey script approach and requires using GreaseMonkey addon).
Now that I think of it, if there were something unique about watched vs not-watched results it might even be possible to have Adblock Plus adblocker disappear the results in part or completely with an element hiding rule, but if at all possible, and to what degree would depend on what the difference is (if something as nice as an additional class attribute on watched results would probably work). Just thinking out loud on that for now, because that approach requires some very specific uniqueness on results that doesn't exist from what I can see.)
TLDNR; Can't reasonably be done unless I missed something obvious.
