The following are general guidelines you need to be effective in this particular space:
>> A good web design that is not too complicated or overwhelming.
>> A good product
>> A USP or Unique Selling Position
But these are not enough to market, let alone sell products online.
As a copywriter, it is your job is to have your visitor want to stay and respond to your calls to action on the website. Introduce your product the best way possible. Persuade them and make them want to purchase.