Why am I recieving error code 107, "Category is not valid" when I make an AddItem call? I know that the category is valid via GetCategories, so what is wrong with me request?
This typically means that you are not making your calls against the correct site. Doublecheck your headers and make sure you've specified the SiteID for the site which you intended to list the item with.
Remember if you want to list an item on the US Motors site, the siteID is not 0, but 100.