Is it mandatory to have an account on the wccm.org website to buy items on the WCCM Store?

Absolutely not

If you already a wccm.org member and you are logged in the website shall recognize you

If you are already a wccm.org member but you have still not done the login your email shall be recognized by the system when you will input it during the purchasing steps logging you in automatically

If you are not a wccm.org member the system shall create automatically a new membership for you using the data you will input during the purchase steps. After having completed the purchase you shall see your new membership credentials on the concluding page. Besides it a mail shall be sent to you with the data you'll need to use your new account