The first thing I ran in to was the article on Advanced Trading from googling Margin Trading. This was pretty worthless to me, so I searched the wiki directly and stumbled on the Market Guide page about Margin Trading.
The Margin Trading skill reduces the amount of ISK one has to reserve when making buy orders on the market. The money is still required when a sale is made. If someone fills the order and the buying pilot does not have the ISK to cover the transaction, the transaction is cancelled and the rest of the buy order is cancelled.
Per the other answer, it is confirmed by this post as well.
So ended my adventure into the EVE Universe. Good to know the wiki doesn't suck :D
Oh, just for fun, since you mentioned intergallactic debt consolidation. This is also on that page:
It is not possible to set a
single buy order whose value exceeds
what you can currently afford.
However, with the right skills trained
you can set a number of smaller buy
orders whose total value is more then
you can afford. Doing so can be
dangerous, since if the market
attempts to buy something and you can
no longer afford it then you will be
fined 10% of the total value. Until
you have fully paid the fine, you will
be unable to put in new orders for
anything on the market. Note, however,
that any orders you had already
entered will still be active, and will
most likely incur additional fines as