This is a two-part answer. Part one: How do you stay safe online?
First, and this is really important, use safe Internet security practices to make sure you are at the site you want and that you have a secure Internet connection. (See, How does Internet security work?) Also, never enter credit card or personal banking or investment account information on a computer that is not currently protected by anti-virus and anti-spyware software. Finally, it’s probably not a good idea to enter credit card or personal banking or investment account information from a public hotspot. (See, Are wireless connections at hotspots safe?)
Part two: Can you trust the online retailer? See, How do I know if I can trust a Web site?