It's where retailers can list their surplus inventory and get biddings/offers from other businesses or individuals. How do I get started and is a shopify website good enough to build this?
If you know shopify, then yes. Use whatever can get you there fastest to present something to your first users. Once you build the traction you're after, look towards setting up a better tech solution that serves user needs with a clean user experience.
B2B marketplaces are inherently different, but not more complex to market. To start, you might look into buying some of that inventory yourself or doing things manually for a bit to evaluate demand.