What is the right payment solution for a B2B marketplace?

I think you can add direct bank transfer option to your marketplace account and act like escrow to provide safety for your buyers.

I don't know the legal complexity for the same but seems something which could be done.

Using crypto currencies might be helpful

My suggestion is to work with one of the biggest, most secure payment processors in the world. It's a short list, but look at companies like ChasePaymentech. You need to be wary of looking like you're laundering money for your sellers. You need to pick a single currency. And you probably will be more expensive than letting merchants bill directly. But handling the administrative burden well, builds trust with your buyers.

This is a discussion that requires involvement of your accountant, your attorney, and also the payment processor. Your processor should be able to help a lot in guiding you to warming up your account. With any processor you can't go from zero to tens of thousands in a month. It's not allowed because of the potential for fraud.

PayPal and Stripe are not strong enough companies to represent legitimacy for B2B buyers. Use a business-respected processor. PayPal and Stripe are for solo-preneurs selling crafts on the street, not for business transactions.