No, it is not allowed to resell WhatsApp for Business. Reseller use cases are prohibited directly from WhatsApp's side. If you sign up for WhatsApp for Business and submit your use case for review, reseller use cases will be rejected by WhatsApp.
All customers that we onboard for WhatsApp for Business must be direct customers of MessageBird.
If you'd like to manage your clients' WhatsApp accounts, they will all need to request access separately and then grant you access to their MessageBird accounts. This way, you can configure the setup for your clients, but their use cases will still be individually approved by WhatsApp.