I have seen way to many callout threads lately... So if you want to buy/sell/trade something on NS, here is some general advice...
Signs of a good seller: Good feedback, lots of posts on NS, good karma points, good communication, and thorough descriptions of what they are selling (PICTURES), etc. They will also be more than willing to give you an address, email, phone number, and people on NS who can vouch for them.
Signs of a bad seller: no feedback, or bad feedback, a new account with no previous posts, etc, No friends on NS, vague descriptions, no pictures, anger, poor grammer, or spelling, vulgarity, etc.
How to be a good seller and generally not piss people off: Follow through on all dealings, don't be a dick, don't lie, ship promptly, get SHIPPING INSURANCE. Double check addresses, be thorough with your descriptions and pictures. Don't "forget" to mention a defect, issue, etc. till the item is already sold.
How to be a good buyer: Be prompt, polite, and thorough when you ask questions, via PM, or thread post. Don't back out on people at the last minute (I have done this... and its not cool), if there is any doubt as to whether you can/will actually buy something, be honest... Give specific dates, and amounts.