This is a rather good idea. Some thoughts about this idea:
The basic of your idea is "trade" your current (stacked) items to new items, which are sold only for a limited time. As you also said in the previous comment, it is mainly aimed at those wealthy players. So I guess this will end up in a situation, that most of those new items will be in those wealthy players' hands. I think that some of these new items should be made affordable for average players too, e.g. an average player like me simply can't sink 100m worth of items for a cosmetic item.
The shop's items hasn't also been decided, but I think beside the cosmetic items, there should also be something items that would benefit skilling and combat (items that would give you a small advantage in skilling/combat). Also, there should be tradeable and untradeable items.
And, what if the wealthy player doesn't want to sink his items, and waits other to sink theirs, and then he can gain an advantage from this? I think you can't force a player to do what they don't want to do, expect if it is against the rules.
(Lel, just some thoughts. Grammar sucks, English sucks, dunno how to give constructive feedback... hope you understood what I tried to say)