I need the whole list of Oracle apps APIs that are used for inventory and purchasing order modules.
The APIs you have access to depend not only on the version of Oracle Applications you have, but also on your Patch Level for specific areas.
Oracle Manufacturing APIs and Open Interface Manuals will give you all you need for Inventory and Purchasing. Metalink and Technet will be able to provide you with updated documentation based on patches. You should always look for new documentation on APIs, even if you have used the same API dozens of times. You never know when Oracle will give you a new and improved tool.