Safest way to trade ?

Hi to all of you,

I just bought an ARC SP-11 mk II preamp here on the AudiogoN market place and i was wondering how i do have to proceed to be sure to not lose my money, let me explain.
My seller hasen't any feedback yet, the add was very minimalist, there was no photos, but he send them to me on request, the price is very cheap. Also, my greeck seller says he's a 71 years old and don't have the knowlegde and tools to open it and send me some picts of the inside of the preamp.
I am sure that most of you would have past their way after having reading the add, but i have the 'feeling' that this seller is telling the truth. Altought, the experience i have into buying audio products, tells me that i have to be caution and to find a way to insure my money. Paypal doesn't cover the risk if the product does not conform to the description, which is in fact my major problem. The preamp is supposed to have been serviced and retubed 2 years ago.
So what i am looking for is a 'middle man' service or something like that. A third party, that both of us would agree, and that will keep my money safe but also insure the seller that he'll get he's money once i'll have received the preamp and checked it.

If you have any idea, suggestion, or advice please let me know, thanks.