You're buying some item. You're in New York City. Other websites sell it for $20. You go on Amazon.com and see that it's $14.99, here's where #1 comes in. Why is it so low?? In our scenario, let's assume "Fugazi FBA seller" just shipped in a bunch of counterfeit merchandise into Amazon's warehouse in Tennessee that cost them $5.00, $14.99 is a major win for the seller.
Now, Amazon matches the price at $14.99 and you buy it "ships and sold by Amazon". This is where #2 comes in. You're a Prime member. You expect to get it in 2 days. You're all the way out in NYC. The closest warehouse that has Amazon's legit inventory is in California.
Amazon says, "Hey we need to get this customer his/her item in 2 days. It's way FASTER and CHEAPER, if we just pick one unit from Fugazi FBA seller's stock that's in Tennessee and ship that out." - #2 and #4.
Once this product is end-of-life or Amazon can't source it, #3 comes into play and Amazon stops selling it "ships and sold by Amazon" and sellers (legit and illegitimate) are left to fight it out.