Over the years, I have purchased many items from various internet forums, web market places, etc. I have done even trades with some people also. I use the following methods to help limit my exposure to a "bad" deal:
First, I attempt to see if the person offering the item has a history. By history, I mean is he a first time poster or has he been a long time participant in the forum. I also try to search to see if he's an active participant in other similar internet forums, what type of participant was he, etc.
Second, I correspond with the person. Does he know what he is saying in the correspondence. Does he volunteer his phone number for me to call him to discuss. The important thing here is to establish a comfortable feeling about the potential transaction.
Finally, would I be okay if the transaction goes wrong. In other words, can I afford to lose the money. If I can't, then maybe I shouldn't be buying it.
It really boils down to feeling comfortable about the potential transaction. If I'm not comfortable with it, then I probably won't go through with it. It comes down to a "gut" feeling.
Bill
Smithville, NJ