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Vendor non visible items do not get reshuffled from my understanding unless vendor is killed.

I have mined a lot especially when doing tailoring. anyhow vendor unseen inventory is from my experience is set. A weird thing is when you mining a merchant you will occasionally have someone else come sell something you cleared out already and it will pop into the top slot of the empty merchant window. I have no way to explain this behavior unless the merchant keeps track of what item where in what order in the hidden inventory.

Remember also that when you merchant mine you will not see the top hidden slot you just cleared if someone else is selling to the merchant as same time as you cleared it. you will close the interface and reopen (assuming same person is still selling ) and you will see the same list of item again minus the "empty slot" you cleared.

Hope this helps