Buy from reputable companies. If you and your friends have never heard of ACME Widgets, don't take a chance. Find out if anyone you know and trust has done business with the company. Legitimate online businesses have their own web sites and secure online ordering processes. They will also provide you with a purchase receipt that can be printed out or will be sent to you in an e-mail.
Online auctions are dangerous places.
You'll soon be bidding beyond your means. Consider using an escrow service while making your transaction. These neutral parties will hold your money, for a small fee, and only release the funds to the seller when the item is received and deemed to be acceptable. If the company doesn't provide phone numbers, a mailing address, and a web site, dump that e-mail in the trash and move on. Blow the whistle.