Here are some of my thoughts:
1. Always use PayPal if possible and buy from/sell to verified members. It's not foolproof, but knowing that a member is verified is at least something. If it's a high-dollar item, the seller may ask you to pay the surcharge (up to 3%) or split it with him/her. It's worth it.
2. If using a money order, USPS ones are the way to go. It's a federal crime to misuse them, and the Postal Inspectors take these things seriously.
3. For an expensive item, ALWAYS do due diligence. This includes: a) Getting a HOME phone number for the seller (if they have one) and looking up the number online to see if it matches with the other info you have (location, etc.). Call the seller once or more to discuss the item so you can get a feel for them. Are they selling a specialized item but don't seem to know anything about it? Maybe it's stolen, or doesn't exist! b) Google the seller's name, e-mail address, and anything else you can find to see what shows up. You might be surprised.
4. If selling an item that was paid for with a money order, always make sure it clears before sending off the item. There are plenty of scams with money orders and cashiers checks being fraudulent. Just because your bank deposits it without issue doesn't mean it's legit (I learned this the hard way years ago when selling a computer).
5. If you agree upon something in a call (see #3), always follow it up with a written e-mail detailing the agreement and get a response from the seller/buyer that he/she agrees to the terms. That way you have something in writing to look back on.
6. For any high-value item, as a seller always ship using trackable services and require a signature. Follow-up right away with an e-mail asking if everything looks good. Assuming so, this gives you some protection if the buyer comes back in a week and says something is wrong.