The lowest effort that I can think of that will easily net you that amount is put in 4 million bids on the purple snipe set. Convert those to another PvP set (Health, Defense or Armor are guaranteed sellers). Sell for 7 million if you want near instant profit. Sell for 8 million if you can wait a day. Do two of those a week and done.
There are a ton of ways, but this requires nothing but the initial investment in the IO, a few converters, and the inf to post it on the market.
You could bid lower if you don't mind waiting, or even pick some of the other PvP sets for diversity. And sell higher for the same. But I do this on pretty much all characters so when I log into them I have a guaranteed profit waiting my return. It's not my primary money maker, but it certainly adds up, and takes just a few minutes when I do log into the neglected characters.