I have been scammed once by some seller from the US on eBay (The item was not as described and the seller never responded me) but I am not saying that all the US sellers are scammers.
I think it is a given that scammers know no boundaries. They can be anywhere in the world. It's the job of the shopper to be smart and see that something is almost too good to be true. Sometimes though the scammer does manage to work his/her magic.