Set a sell order for a rare item at 1 billion ISK.
Set a buy order for that item at 1.5 billion ISK.
The victim sees the buy order and buys the item from you at 1 billion ISK to fill your buy order. But when they try to sell to you your wallet is empty, the transaction fails, and they’re left holding an item they paid too much for and your alt gets the 1 billion ISK. The scam depends on the item being rare enough that they can’t just get it somewhere else, but even if they do the buy order still fails and you just have to reset the scam without making any profit on that attempt.
How is this a “scam”?
Because the more appropriate behavior is for market orders that can’t be filled to be removed from the market, or at least temporarily disabled and highlighted as “not available right now”. It shouldn’t display as a legitimate order if it can’t be completed.