Q:
The back items and transmute items in search
There actually is a filter for back items and also skins (and a few more other things), but for some reason they don’t list them in the filters you can select. It would be such a trivial change to add, yet a year later and they still haven’t done it.
Back items used to be hard to get, since they were only available through the personal story. Then they through some tradeable ones in the game, made them mostly for level 80s and also made them the cosmetic centerpiece, whereas originally, they were invisible.
For more specific searching, you’ll need to use 3rd party tools, like http://www.gw2db.com/items/back
There’s actually quite a few TP filters that either don’t exist or don’t work right. The various consumable items are a big example of that. I’d imagine the lack of a fix to these things are the same as the reason for the lack of another critical upgrade: armor by type. It seems Anet just hasn’t deemed those upgrades worth the time investment from their programmers and debug staff. Considering the performance issues that still crop up here and there with the TP it could be tangentially related to that as well.