Carro by the developer VYRL is a top-rated Shopify app that enables Shopify merchants to cross-sell products on multiple Shopify stores. In other words, it is a cross-store partnership management app. It most likely works like dropshipping, you can find and source products from thousands of Carro partners. With Carro, you can partner with top Shopify stores and list and sell their popular products on your Shopify store without having physical inventory. Additionally, you can also ask other Shopify stores to sell your products. 

You can list products for free but need to pay a usage fee on a per-order basis.