I've done some extremely large sales over the past few months, including a couple over the 10 digit mark. You have a few options really.
In the instance that either I know the buyer/seller, or if I know for certain that they are a trusted member of the trading community, I will simply toss a handful of apples on a vendor for 100m each and hand the item over once they are purchased. Definitely need to know who you are dealing with though so you don't make a huge mistake. Double check through ICQ or PM's that you are actually handing the item to the right character and so forth.
Safest option is obviously to use one of the trusted brokers out there. Some of which even offer their services for free. I've used a couple different people in the past that myself and the person I was buying/selling to agreed upon. All of my experiences were great with those I have used.
Third, but less popular option in my opinion is the item value trading method. Have the person you are selling an item to buy X amount of Y item or items. Say 10 xfer tokens at a 40m value each traded via the normal trade window for an item that is being sold for 400m. While I don't think this is a completely bad method, I hate dealing with items that sometime change in value quickly. So there is a tiny bit of a gamble involved.
I'm sure there are other ways out there, but these are the ones I'm familiar with.
Oh...and you can xfer a LOT more than 300m even via shard shields.
Shhh, tis a secret! heh
