The first question isn’t clear as to which of the two “Discrepancy” reports are being referenced, so I will address both.
e-foro; Making eCommerce Easier
Back in the day I made a modest amount of money selling books online. Comics, paperbacks, new and used; that sort of thing. The biggest headache I had at the time was keeping track of my inventory across multiple platforms. When I sold something on eBay, I had to quickly take it off Amazon. If I sold it on Amazon, I had to quickly take it off eBay. And if it was a hot item, invariably I would double sell.