I think prices are sustainable, especially the Japanese market which is spreading internationally at an accelerated rate. English I have less faith in as big of returns in the short term. How many collectors in 5 or 10 years will look at their English copy of a card and desire a higher quality Japanese copy? It's impossible to say. But long term I think English sealed and a few graded singles from each set will gain value.
I tend to feel the same: I have a lot more confidence in the growth of the Japanese market as international collectors gain more interest in the “original” versions of the cards.
