This article explains how you can enable the 'Buy Now' button on the product listing where users can click on the button and proceed with the payment. 

To activate this function for the marketplace, here are the steps you need to follow:

The item's owner must has permission to sell listing on Marketplace

• You can configure permissions In Admin CP> Apps> Marketplace, choose a group of user that you want to allow sell listing

• Next, search the setting "Can sell items on the Marketplace " and set it to Yes

Seller's PayPal Email is a must have component in "Account Settings" section

• From drop-down button next to avatar photo, click on Privacy setting and select Account setting section. Paypal email and Merchant ID are required to be filled in under PAYMENT METHODS block

Enable Instant Payment 

• When you create a listing, don't forget to enable the Instant Payment to yes, or you can edit by clicking on the gear icon on a specific Listing Item > choose Edit listing> Enable Instant Payment

Now you already finished steps to add Buynow button to Marketplace listing.This is how it will look in the storefront.