This is quite a comprehensive list, and I can very much related to the points raised as I am building up a B2B marketplace of my own. One other point I would like to add to the list is: Community.
Be it a B2C or B2B marketplace, one of the key challenges in readying a marketplace is to build and rally a community. From a sociological standpoint, this means that the marketplace does not merely provide a functional benefit for its users. Instead, it fosters an identity and purpose that users associate with, to an extent that it creates a feeling of belongingness.
An idealised AirBNB community for example does not just provide a room, but instead a different experience of staying with locals. A used item marketplace like Carousell does not just afford the resale of used items, but instead you are aiming to create value for your used-but-still-in-good-condition items among other like-minded individuals.
For would-be marketplace developers, spare some thoughts for the ideal community you want to build. Then head out, talk to these people. It will help validate the assumptions too.