There are two sceneries.
1. You have a physical store or own product and you want to sell online as well.
2. You just want to resell products from other vendors.
In first case you need a website and thats it. You will have to think about payment options and shipping etc.
In second case it is same but when order arrives you need to get product from the vendor and ship to your customer or have a agreement that they ship product. There are many additional issues like inventory and price control, shipping times and faulty products.
There is no need to assure minimum sales as they do not have to do anything.