Can I let smaller agents sell through my white label travel portal under their own brand?

Yes, in most cases you can let other agents sell through your white label travel portal under their own brand, although how far that branding goes depends on your plan.

The usual setup is that each agent you sign up gets their own login and their own branded front end, with your logo replaced by theirs, while the supplier connections and the main account stay with you. You remain the one holding the supplier relationships, and the agent sells from whatever you choose to make available to them. You can normally decide which products each agent sees, and you can set your own margin on top of their bookings, so you earn on everything they sell while they still add their own commission for their customers.

What varies most is the level of separation. Some plans allow a fully separate web address and separate branding for each agent, while others only give them a branded login inside your portal rather than a public website of their own. Payment handling varies too. Some setups let each agent collect money from their own customers directly, and others require the agent to book against a balance or a credit limit that you top up and control. Those are commercial decisions as much as technical ones, so they are worth settling before you start signing agents up.

If you are planning to build a network of agents, write down roughly how many you expect, whether each one needs a public website or only a login, and how you want the money to flow, then check those points against your plan before you promise anything to the agents themselves.