re: "Clients telling me to pay for training materials. Is this a scam?"
Go to a restaurant.
Tell the waiter that you will pay for dinner, but the chef will first need to purchase a cook book from you.
See how that works out.
* * * *
When a client pays you money, that's a real job.
When a "client" asks you for money, that's a scam.
It's not that complicated.