Customers are already present in the .NET API as writable entitles.

Inventory items are already present in the .NET API, but they are currently read-only. So, as long as you aren't trying to enter, edit or delete inventory items, you should have access to these things.

Yes I really need to modify inventory items with the .net api.  Is this going to be added?

I have the same issue, cannot create items through the .NET SDK. However I was reading on the forum and the only way to do this is through the COM API (unfortunately).