You guys want to know the best way to buy isk? $20 per billion, more than double what you get with plex...
Go to gamestop, buy a $100 gift card. Go to the
http://forums.d2jsp.org/forum.php?f=233 and post it for sale, the going rate has been roughly 1000fg/$10 for years and many people buy them. Sometimes they take a week or 2 to sell, but if you lower the price a bit I'm sure it would get snatched quickly.
After you mail the card and get your 10,000fg you can then trade it for 5b from someone here, possibly more if you find the right person.
I've done this a dozens of times, though I was always the one paying fg for the gift card.
And like the others said, there's really no reason to buy fg from the site. I get most of mine through selling wow gold, where I basically owned the market on the server I played on... until I quit recently when I heard how the economy in eve was setup and it just made me drool.