Any suggestions on how to manage my client's Amazon store?

There are 2 major skills one must have:

1. Amazon seller knowledge.
2. Product research knowledge.

Both can't be acquired by a simple inquiry.

I highly suggest that you or your employer enroll in an Amazon Seller Master class. It will be hard for you and your client to accomplish these tasks if none of you have the basic training. It is also optional to just hire a VA with Amazon Training if it's urgent.