While this story is about Craigslist, it can apply to ANY internet method you use to sell or buy your items personally.
If you have something expensive, DO NOT tell the people your phone number or street address!
DO NOT meet them alone or in a remote place, even if you are the buyer. (They know you have cash)
Tell someone else the contact information you have regarding the others. Disposable e-mail addresses like Gmail, hotmail, etc., don't really tell you or the authorities much, but it is something.
DO take a cell phone and be talking to someone when the others arrive AND have someone else with you. Tell your friend the car information, description, maybe even license plate of the car they come in.
Make sure the meeting place is in a very public, populated place. Even if it has high traffic, don't meet in the far off corner of the parking lot. Use good common sense.
When you leave the strangers, don't go straight home. Go into the store or drive to another well populated spot and get something to eat or drive to the local police station.
ESPECIALLY if your "gut" tells you something is not right. Even if you made the sale, you now have their money and they might want it back.
I know we have a friendly community here, but you do not REALLY know who is coming, from where when you advertise on the internet. Here is the story. http://abcnews.go.com/print?id=10568167