You can have some really amazing products available at the best prices around, complimented with some really top of the range photographs of those deeply aesthetically pleasing items as well as a fantastic blog with amazing entries added every day, but that would all be for nothing if you don't have a user-friendly interface. If it's hard for people to get around your site and to figure out how to order things, they're just not going to bother. So how can you be 100% sure that your site is good for its users?
Well, have you ever thought about going onto your own site and trying to order something? Try to experience it from the other side. Even better, you might also try asking somebody you know to try and use your site to buy something. If they want to buy something, great! You could do a whole test run, but if they're not really interested in your products just pay them back and don't bother to send it. As they go through the process, you can ask them to make notes on how the site could be improved. Ask them to be as honest and frank as possible - you could gain some really valuable insights through this process.
Hopefully, when you ask somebody to take your site for a test run, they'll tell you that it all works perfectly and doesn't have any issues, but it's always good to be sure. You might also like to provide a place for your customers to give you feedback, either through a comment section or just a form in which they can confidentially enter their thoughts. This kind of information can be really useful and taking steps to obtain it and to act on it could help to push you ahead of your competitors.