Report any suspicious activity on your credit card immediately. I like to deal only with with firms that state that they only charge your credit card when the items are shipped. Have all manufacturer's data available when you order- item name and manufacturer's item number. If you are not sure that the item you are ordering is exactly the same as the one you want, don't order it. Use the manufacturer's website to verify that the items you are ordering are compatible with your system and needs.
Most sites post a page of company policies including details concerning return.
Find and read it carefully and completely. Don't do business with a site that does not post these policies in detail. If you need the assistance of a salesperson to help you decide what you need, and you want to be able to get advice and help after the purchase then a retail outlet is often the best choice for you. If it is computer goods you are looking for, take a look at these three sites.