Well, suggestion #1 is to use an official middleman from D3Scene here.
#2 : If the middleman rips you off, then you both are screwed and the seller may be able to re-gain possession of what they are selling depending on what they are selling. The buyer is SOL UNLESS the money was NOT sent as a gift via paypal. If it is sent as a gift, then there is absolutely nothing you can do about it. If it was not a gift then you can open a dispute to attempt to get your money back.
Hope this clears some of this up for you.