Free offers that aren't immediately fulfilled. You should make some kind of free offer on your site that will allow you to capture visitor names and e-mail addresses. But you should only do this if you can immediately fulfill your offer. Many sites offer free consultations or information, then fail to deliver. Non-secure or confusing ordering procedures.
Many sites have non-secure or confusing ordering procedures.
Better to not request credit card info, etc. If you have more that two or three items for sale, invest in a shopping cart ordering system. Most people will not take the time to load plug-ins to view or do something at your site. They'll just click away. JavaScript pop-up windows can be annoying.