Inventory management

Hi guys, I’ve been starting to branch away from FBA and moving to other platforms such as eBay, Etsy and Shopify (and FBM). I’m struggling to keep track of all my inventory and don’t want to oversell on different platforms.

I’ve looked into veeqo, channel grabber ect. But they are all £100+ per month and I’m stuck really.

Any help would be appreciated thanks!