(1) look up the seller --- see his previous posts and dealings with other members, check out how long he has been a member and his reputation level
(2) consider the other aspects of the transaction --- shipping, customs duties, insurance, etc.
(3) make sure all your questions/doubts about the item are clarified and answered to your satisfaction before making the deal final
(4) don't risk anything you can't affford to lose
(5) use payment methods like PayPal where there are buyer protection policies
(6) know how to contact your seller outside of the forum --- ask for his telephone number or retail store physical address or a second email address --- most legitimate sellers will be glad to give you their business information
Although diyAudio strives to make the forum a safe and secure place for its members, it is always wise to err on the side of caution when buying stuff over the internet.