Typically when I have items I want to keep I lock them by adding them to a pet like so:

(this is just ONE of my storage pets) but that means if I want to use the items I have to find them on the pet- you can kinda see how that's an issue ^^;; I either end up have to take everything off and hope I don't miss an item in the reapplication, or spend quite a bit of time digging for the item or two I need. I don't dress up my pets very often, but when I do this drives me nuts!
What I think we need is the ability to lock individual items: 1: I imagined it as a tiny icon in the bottom right corner of the item in the shape of a little lock, grey and partially open for unlocked, and then turns gold and closes for locked items.
2: Alternatively, considering some items are SUPER tiny, a "lock this item" option under the item name and amount available. This would help give players some control over what items they have appearing in trades without needing item cows, over-detail rules, or continual trade editing, keeping the item free for player use! =3
For people who trade item doubles or want to keep 3 of, for example, 5 of an individual item maybe let the number locked be set, but I assumed that be much more difficult and didn't want to over-complicate the idea too fast. =3
A third alternative is customization item grouping, so you can put all items you want to lock into a group and lock THAT, but I imagined that to be a much bigger change than adding a small lock option. (I could be wrong, as I have absolutely no coding or programming experience. I'm lucky any given day my computer doesn't implode. XD)