Posse Inventory + Merchant Interaction
It would be easier to also have your in posse inventory in the merchant screens, rather than getting (mostly) the junk you stored in your posse's inventory. Maybe when interacting with the merchant (mostly the general store ones), your inventories get merged and exceed the x5... or just an option to "sell all junk, including your posse members," you could even extend this to your saddlebags (or is that to much immersion break).
With the others (weapons, amulets, potions, vests sellers) it would be less cluttered if your inventories will temporarily merged when selling. And they don't really cause that much problems, since most of my inventory at least is mostly junk.
*But things that they "owe" like deputy stars, faded photo, should be protected (in case other people don't want to sell them).