Try sell the most popular things in the world, thr indemand thigs, Like cellphone.You can have a great profit in it. But if you sell make sure that it is lesser than the mall price, The cheaper the more customer comes.
Yeah something like that would give him quite a good success rate if he sells the things a little cheaper than the markets and for that he would have to get his things directly from a company and if he will be able to do that then he would be able to make a great online store i'm sure.
Well if op is going to build up just like an amazon type of online shop and he is going to sell everything in demand for sure it is going to have a tough competition for him.
Because as of now, people tend to trust more about the online shops that are already in the industry for a long time.
But still if you are going to have a different marketing strategy for sure you are going to be well known for having a unique online shop.