First, check if the person has feedback in the feedback forum. Some people here accept paypal, most don't.
Usually, after a trust has been formed, through communication before an agreement is made ... money is sent first then item is shipped (Money order or check).
When I bought my item from someone here in the forum, he was very informative, well known personally as well as just a member AND he gave his Drivers license # as well as other info. When you send a M.O., make sure to keep the receipt just in case you have to make a claim.
Before anything is done, try to feel the person out. I was almost burned on a starter box from one person, luckily I backed out just in time. Follow your gut. Any grain of doubt, back out of the transaction. That's all I can say. Everything is a risk.
Not to put down some of the members, I'm sure they are honest BUT as a rule of thumb, if it's a younger seller ... I would make sure a phone # is given and the parents are made aware of the sale or trade.