Than Once Many people believe cookies are an invasion of their privacy. If you must use cookies, don't nag your visitors more than once per visit to set a cookie. But you should only do this if you can immediately fulfill your offer. Many sites offer free consultations or information, then fail to deliver. This can permanently damage credibility.
Many sites have non-secure or confusing ordering procedures.
Better to not request credit card info, etc. If you have more that 2-3 items for sale, invest in a shopping cart system. Make it easy for customers to buy. Most people will not take the time to load plug-ins to view or do something at your site. They'll just click away.