Seems Arkham (quoted below) addressed this in the same way I was thinking about it. If the inventory is separate between them anyways, what's the point of sharing the retainer in the first place?
I'm thinking along the same lines as far as limiting how many items we can have up for auction at a time, at least if it's correct to interpret the translation as inventory not being shared. And/or to limit the amount of unique names on retainers? There's got to be some alternate reason...