I'm kind of confused about when this is needed?

For example, if company Z is looking for product A and they are located in California.

Do I need a resale certificate if I purchase product A myself after locating it and then have my supplier (aka the company that has product A) send it to company Z directly (after adding some profit myself to the initial cost)??

Company Z is technically buying it from me correct? Even though it is another company that is basically supplying the product.