DarkSoldier said:
Only give that kind of information when you see the lock in your browser's system bar, and never through e-mail, because who knows how many "official" eBay and Amazon e-mails are actually spoofed to harvest unsuspecting schnooks' personal information?
The lock symbol on your browser bar is meaningless- it means the website in question is using https, that is, "http secure". To do that, the company has to have an electronic certificate which is used to ensure encryption of the transmission between you and them. Such a certificate is available from verisign. You need a hundred bucks or so, and a mailing address, last I checked.
So basically, the little lock symbol doesn't mean that the website isn't a scam; it means that if it is a scam, the scammers are mildly technically competant.
There aren't really any technical means of ensuring that a company on the internet isn't a scam. Use the same standards you would offline- is it an established company? Have you heard good things about it, or do you know any friends that have used it? Someplace like Amazon is probably going to be ok. Random sellers on Ebay, it's at your own risk, and someone soliciting over email, definitely a no-go.
And, as a side note, use a credit card, not a debit card (even one with a credit card backer), online- credit cards have a liability limit, and the most a thief can take you for is 50 bucks if your number gets stolen. It's much more of a hassle to recover funds if someone gets ahold of your debit card number, since it is gone, straight out of your bank account, and there's no fraud liability limit on debit cards.