It would be a horrible idea and here's why.
Imagine spending two hours searching for an affordable listing of a specific item you want, but all you find are high-end prices. That would be because some "entrepreneur" already foresaw demand of said item and proceeded to clear out every listing in every trader location that was priced well, and then most likely re-list at a price you really don't want to pay.
I am guilty of this as well, but you know what? It became way too much of a hassle, having to spend five to twenty minutes per trader just to MAYBE find a great deal. It wasn't worth it at all. If there was a specific search function I can tell you for a fact that I or someone else would have already snatched up that bloodforge helm you wanted to find for 26k. And it would instead be listed at 48k, and most of you are asking ZOS to give the sharks the ability to do this. It's not a joke and casual players would honestly not be able to afford anything that's new and shiny.
Not to mention it would take away that adrenaline rush when you finally see the first few letters of that motif name flick by your screen after searching desperately through countless traders in multiple trading hubs. Sure this would make our toons lives a little bit easier, but it would indirectly make them harder as well. So please stop asking, you really do not understand what it is you are asking for.. and the grass isn't always greener on the other side.