Knowband is described as 'Avail feature packed Magento, PrestaShop, OpenCart and Shopify modules and extensions for eCommerce site in a seamless manner through Knowband'. There are six alternatives to Knowband, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, Magento Community Edition, Windows and Linux. The best alternative is Skubana. It's not free, so if you're looking for a free alternative, you could try Debutify. Other great sites and apps similar to Knowband are Apptha Marketplace (Paid, Open Source), Module Bazaar (Paid), Softprodigy (Paid) and CedCommerce (Paid).
Skubana is an all-in-one operations management solution covering order fulfillment, inventory, purchasing, analytics and more, across multiple channels including warehouses and online services like Amazon, eBay and Ratuken.
Apptha Marketplace is a readymade b2b/b2c Multi vendor script built on Magento 1 & Magento 2 and it is cloud ready as well. Packed with 3 free add-ons (Social Login, One-step checkout, Super Deals) the script offers 1 year free cloud hosting on Amazon Cloud environment.