Let's take it a bit at a time:

"If i buy some books online or at a store to sell and I pay immediately, that's a purchase for me, so to properly record this, do i use...

- Vendor > Enter Bill" <== You already paid. There is No Bill that needs to be entered, and then Paid Later. You Already Paid.

"and when i purchase something and pay immediately, I go directly to Write Check"

Write Check = paid directly from Checking using paper check, paperless, debit card, EFT, ACH, etc.

"or Enter Credit Card Charge ?"

If you put it on the Card, sure, you need to enter how you paid for it = bought it by using the credit card.

"If so, when do you use Vendor/Nonprofit path?"

Menu = words at the top. Vendor Menu = vendor tools. NonProfit menu = a subset of tools grouped together for convenience, that show as if they are specifically NFP functions, but they are not. it's just an Extra Menu holding a set of the standard tools seen elsewhere.

"and then when i sell the book to a customer, do i use...

- Nonprofit > Receive Payment (Customer Payment window) ?"

If you Made the sale by listing it on an Invoice, you will Receive Payment to show the invoice got paid, when it gets paid. If you also got paid right then, that is Sales Receipt. This is from the Customer's menu, because it relates to Customers.

Purchases are entered for who and when you paid, from which source of resources: Checking, Cash on Hand bank, Safe Bank, Credit Card. Or, Bill = to be Paid Later, such as an Auto Parts store makes multiple daily deliveries to the Auto Repair shop, and then sends a Statement at Month End for them to Pay later. Each delivery is Enter Bill. The payment against the statement is Pay Bills, selecting all the deliveries that now are being paid for.