When SE do let you have a 3rd and maybe a 4th retainer (later down the road), they disable the ability to sell on them. Selling 40 items is enough from your 2 retainers, they're no need for players to be able to sell more. My reason for this is so we don't have RMTs selling on that one account, sure it's not much a difference overall but at least it won't clog the market as much. More so if they hijack multiple accounts or made more accounts.

If by some chance SE don't give us a 3rd retainer I would settle for a mimic chest that can be summon by you for when you're doing crafting and need it to hold the extra mats. Or a golem that hold more items like a retainer but it can't sell.