They feel comfortable by the name of the store, or because they can actually see the store, the workers and how it runs. Anyone can build a web site and start a business. That is why it's so easy for someone to scam people out of money. You have to be careful because you really don't know who the person is that owns the site. It can be someone that will take your money and run.
Most people are very cautious about buying things on the net.
To make your customers more comfortable about buying your service or product, you can do the following. If you want to become successful with anything, you have to stand behind your product or service. People want to get their money back if they are not happy and you can't satisfy everyone. If you ever have someone that wants a refund, you have to stand up to your word and refund their money. The guarantee on your site isn't worth anything until you live up to it.