I suggest doing your best in the war instead of doing the 10mil. Do your best to gather as much allies as possible and max your boat stats BEFORE doing the war.
You can just redo that part. That's what I did, at least. Much easier than doing the repetitive buy n' sell.
Easy part: Max ship LVL and gather as much allies as possible (strategically route yourself that would use less time as possible, also DON'T forget to save at each islands when gathering allies so you can check if the war hasn't started yet; pretty much a checkpoint).
Hard part: Collect 10mil. Since you can produce your own currency (Pirate coins), do money laundering. Use pirate coins when its value is low. Then halt in using it when it inflates. Then, repeat the process.