I sell more domains to buyers from the US than anywhere else. Also, more non-payers because of that.
Statistically it is hard to say what country has the highest % of non-payers, but I will say that from my experience Western/Northern Europe has the highest chance of people paying. I rarely have deals in that region fall through.
I don't think it is country related but mostly, in general, where you sell the most, is the place where you might see the most non-payers. Having said that, I have seen rich country players do this less often than developing countries although this could not be generalized.