I've realized that "Sold by Amazon" must not always be true Amazon when "Sold by Amazon" items could go in and out of stock at a frequency faster than reasonable and wondered if it was the same stock and real seller as "Fulfilled by Amazon".

Another thing Amazon does these days is real-time delivery time arbitraging on top of real-time prices. You may see an item be delivered by some date X, only to see minutes later it is available to be delivered by date X-2. To them it's another lever to pull.

They must have some game theorists/economists working on this.