a. If Amazon sells a 64 GB Sandisk card
b. Merchants A and B also sell the same card
c. Merchants A and B have opted into the fulfilled by Amazon program.
Question: You purchase the 64 GB Sandisk card and pick Amazon as the seller. Whose inventory did the card come from?
Answer: Undefined.
For semantic purposes we can think of the following mental model - For a given SKU Amazon takes the memory cards from its own inventory and inventory from all other resellers who have opted into the "Fulfilled by Amazon" program (in our example A and B) and puts it together in a giant heap in its warehouse. When a order comes in a card is picked at random from the heap and shipped out to you.