Hi Momo, I think Nembol may help you in the future. All the products you upload using Nembol will be sync'd across Shopify, Amazon, eBay and all the channels Nembol is integrated with (10+). If you sell on a channel, inventory is sync'd on the other, to avoid double sale. And it's free to download and use: https://apps.shopify.com/nembol
Let me know how you find it, I'd be happy to hear your thoughts.
– Simona