There are a wide range of solutions to running an online store.
These range from not really having a store of your own at all (such as selling things on eBay, or through Amazon.com), to a fully customized website and online store which takes credit cards and looks and functions exactly the way you want it to.
There are many inexpensive to moderately-priced solutions in between these.
The more professional the store, the better it will work. Meaning the higher percentage of visitors who end up buying.
But, almost always, the biggest issue is getting visitors to your site in sufficient number.
Across a wide range of industries and products, we have seen a rough rule of thumb of 1%. In other words, maybe 1 in every hundred visitors will make a purchase. That is very rough, it can be considerably higher or lower.
The point is that it takes a LOT of visitors to make a viable online store.
If you want to have a successful online business, the first thing you should do is work out a REALISTIC plan for getting a lot of visitors to your site.