The problem is that most people don't understand that online businesses are really user interface businesses. Sure, it's hard to get people to come to your site -- you need to do a lot of marketing, and you need a good understanding of search. You also need some type of product to sell, but that's probably more a result of search than anything else. You can only sell what people search for, after all.
But once you get someone to your site to buy your product, if things don't look right, they won't buy. This is where user interface comes in, and it's much more complicated than you think. For a primer on user interface, read Edward Tufte. In the meantime, here are a few quick tips to make sure you convert visitors into customers when they reach your site:
Trust online is not as much about the product as it is about the presentation. Easy-to read, balanced content, with color and design choices that convey high quality can actually go a long way to make your site look trustworthy. The more content you have on the page, the harder it is to achieve this look, but many sites do it well, like these casinos online.
Match the customer's need.
Amazon designs its pages so that they look like they were tailored to your search. The products seem to be there especially for me, and the more it looks like the page was built for me, the more likely I am to look around. Here's an example of Amazon's page for Hero Factory Costumes. Amazon doesn't actually have any product that match the search, but I see lots of related, and sort-of-match products that are likely to catch my eye.
Make buying easy.
Don't make people give lots of information, because there's a drop-off with each line someone has to type in. And don't make people type in a password if they don't want to. Tell them how many pages they need to click through to complete the order -- unknowns are no good when your credit card is out. There are so many payment systems out today that aim to simplify the process, which means the selling page can take lots of different forms. Shopify, for example, gives you out-of-the-box user interface solutions for a shopping cart. And MuCash makes micropayments easy for blogs.
Does this look easy? Take a look at Twitter's home page. The company spent a year looking for an in-house designer. That's because Twitter has huge user interface issues and they are complicated. The more information you have on the page, the harder it is to design well for the user. So don't skimp on the user interface part of your company -- it'll cost you in sales.