A random number can be used by various different rules to populate item pools in various different ways. Anything that you can explain out in detail with randomness can be done with a big enough seed - a single floating point real number would probably be plenty for item pools using whatever rule you'd like to interpret.
I don't remember seeing Agnostic Collar in the stores, but I might have ignored it anyway. Agnostic Collar in stores would support the "shift up the queue" theory of interactions between preps and Shop Scroll - Badge is prepped, plucked out of the shop pool, and AC shifts up to a basic store. MW shifts up the queue from "first benchwarmer" to the shop scroll.
I really can't remember whether I saw the collar or not last time, though.