What happens if I get into debt?

Solution 1:

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:

Fines

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 well.

Solution 2:

While it is possible to end up with a negative balance in your wallet by various means (the most common involving having illicitly-purchased ISK taken away from you by a GM), Margin Trading will not allow you to go into the red.

If you don't have sufficient funds to cover the order when it is filled, it will be silently canceled, and you lose the broker fee.

The advantage of having Margin Trading is that you can use that remaining 250K for other things in the meantime, as long as you have it back in your wallet by the time the order is fulfilled.