We have 2 shops on Shopify, We used Synkro to synchronize inventory more 7K products (sync with Barcode/SKU). Works very well !
I commented on this product already from one of my other stores which uses this app. It really has saved me a lot of time, and the developer has been quick to respond to any questions or requests I have had.
We have a single inventory which supply's multiple different Shopify stores. When a product is sold in one store, this tool eliminates the need to manually adjust the inventories in all of our other stores for that product.
It took a little while to configure everything, but once we set it up, it has been working great!