Search eBay right now, and you will see that one of the Japanese PLAY promos is selling for $187. ebirdman is selling his for like $600, which I believe is completely unrealistic. I realize he does not want to sell most of his cards, which is why he generally lists his items for super high amounts.
As for the English version, I believe you would find it difficult to get $400 for a PSA 10. While the card is rare, I don't think it is worth that much. A standard English Vaporeon * sells for around $25-$35. I could see you maybe getting $200 for one, but not $400.
The person you sold the PSA 9 to was probably just tired of searching for a PSA 10 one (or a graded one for that matter), and that is why he/she offered so much for yours. Not to mention that hardly any PSA graded gold stars hit the market on a regular basis, just my thoughts.