Than Once Setting cookies can help you better serve your visitors. However, many people believe cookies are an invasion of their privacy. Some shopping cart and personalization software uses them, and some sites deposit several per visit. Many sites offer free consultations or information, then fail to deliver. This can permanently damage your credibility.
Many sites have non-secure or confusing ordering procedures.
If you have more that 2-3 items for sale, invest in a shopping cart ordering system. They'll just click away. JavaScript pop-up windows can be annoying. This should be obvious, but many people take copyrighted material from other sites and pretend it's theirs. Doing this will eventually bite you and could lead to serious legal problems.