It'd be a lot more helpful if you provided some more info. A 'store' could be one of a number of things:
- An E-Commerce Addon (that would NEVER, EVER be part of the core product)
- An online store full of XenForo resources (aka the Resource Manager which already exists)
- A file store
If you're looking for e-commerce, look at an e-commerce system. It'd be like trying to have a blog built into a contact form - two very different scripts with no need to ever have them built into one package.