Live did some major changes to how item slots are handled. Part of the problem is that it doesn't seem to be used exactly the same way in all cases. So, for stuff like selling and I am sure plenty of other spots, we will need to figure out exactly how they number slots and such. For stuff like item moves, it is 12 bytes used for identifying slots. For inventory/items, I think it is 5 bytes. For selling items, it looks like 8 bytes.
Also, a very useful tool for figuring out struct info is HexVis, which you can find here: