I have to echo Jeff's statement. What is the characteristic of the item that makes you believe it is platinum? Are there markings that are industry standard indicating the item is platinum? Has it been tested? If so, how has it been tested?
Just my opinion, but if the item is not magnetic, it is likely to be either cast aluminum or cast magnesium. Other metals are possible, depending on the purpose of the item.
if you know the weight , You can do the Math Then offer to him for say 90% of value & since he already Tested it & insists it is Platinum, He would be the one who should be willing to buy.
Good Luck ! I think You can actually use this Gold Calculator below for Platinum.
Just change the price of the gold to 957.00 (platinum Value)