As has been said, if you're a buyer neither of the above offer you any protection. For higher value purchases I only pay via BT and ask for a name, address and land line phone number. I then call the seller and make sure everything sounds genuine. For very high value purchases I only pay cash on collection, inspecting the item and asking for a receipt. If the seller sounds iffy I just walk away.
Generally buying from forums means you're buying from fellow togs. If you do a bit of research (how long on forum, types of posts etc) you can get a "feel" for the type of person they are. Plus if they talk on the phone and come across as fellow enthusiasts with a good reason for selling your less likely to run into trouble.
Always though it is caveat emptor. If you get a bad feeling walk away.