To allow transactions without admin intervention, you need to integrate a payment gateway that supports direct payments to the customer's account. This will enable buyers to pay the customer directly for their products without the need for admin intervention.
You can consider integrating payment gateways like PayPal, Stripe, or Square that support direct payments to the seller's account. These payment gateways have APIs that you can integrate with your marketplace to enable seamless transactions: https://marketplace.magento.com/magenest-module-stripe-global.html
Once the payment is made, you can update the order status and notify the customer and the seller about the transaction. You can also set up a commission system to automatically deduct your commission from the seller's payment before transferring the remaining amount to their account.
It's essential to ensure that your payment gateway integration is secure and complies with relevant regulations to protect your customers' payment information.