Solution:
Custom Post Types can add a lot of power, especially if you dress up the Custom Fields with meta boxes. You can always use a role-scoping plugin to limit the access of end-users to only be able to use these Custom Posts and nothing else. CPT are a great way to handle inventory management and not rely on a plugin that will either cost $ or potentially break months down the road.
That said, many plugin you would find would most likely just create a Custom Post Type anyway. Take a look around on the codex for info on creating CPT, and if you’re interested, look at jaredatch’s work on Github.
http://codex.wordpress.org/Post_Types
https://github.com/jaredatch/Custom-Metaboxes-and-Fields-for-WordPress