Been asked for a million times. Never seems to happen. Since there's absolutely nothing game-breaking with such a feature, I assume it must be a programming nightmare to implement. That, or they just don't like us
When it was on the issue tracker, I believe JChan responded to the effect that while it could be implemented, it wouldn't be performant enough to do so.
Of course, I wonder how "performant" it is, compared to running dozens of searches, and hundreds of ID clicks to see which ones you might need
Each of those searches is likely "performant" by how they structured their data. What I believe she means is that comparing each of those search results to what your character already has - would not be. I would hazard a guess that the data returned in the searches does not contain item data that would allow such a comparison, only names and item IDs. When you click an item to inspect it, then it loads that item's actual data. But how that differs from going to a vendor (or bazaar pc-vendor) and having the option of "show me items i can use" turned on, I am not sure. If the bazaar search window had a similar checkbox that would be lovely though.
The problem is, how would this query be structured? Are you searching for all collection items that you are missing? If that's the case, that is a very expensive query because it would be searching across all items from all expansions. They would have to add sub drop down for collection items to limit to an expansion and then another sub drop down to keep them to certain sets. That would cut down quite a bit on searching. However you can already accomplish the same thing just by looking at the achievement window for those sets to see what has/hasn't been collected. Maybe something easier on the UI side of things is just dump a file that compares what you don't have checked on the achievement window instead of hitting the database to check for something like that. Make the client work instead of server side.
I'd be perfectly fine with them lowering the processing priority of the search, and just putting up an emote saying come back in 5 minutes for the results. I could do something else for that 5 minutes, and then wouldn't have to run a stupid number of requests. Another Bazaar feature I'd love to see is a "shopping list". If I'm looking for 10 items which have nothing to do with each other, I have to run 10 searches. I'd love to be able to enter a shopping list, and just have it search for items on that list. Kind of like the favorites for the recipes containers.