I recently discovered Shape Of Dreams and it’s a lot of fun solo, might be fun for you in co-op. Others you might look at:

• Sulfur
• Sworn
• Abyssus

Services like eBay are part platform, part people. eBay’s people do a lot behind the scenes to ensure it runs smoothly and ethically… AND they take a significant chunk of the proceeds. You could put up some free software that does auctions but you wouldn’t have all those high paid security and trust workers working for you.