I'm trying to understand how a partial fill would / should happen at an exchange and how that would occur
For example, if the BTC rate was $10/BTC and
- Someone wanted to buy 100 BTC at $10
The only sellers were
- One seller wants to sell 50 coins for $10 USD
- One seller wants to sell 50 coins for $9.90 USD
Would the final transaction not be completed because not enough matching shares were available? (fair benefit)
Would the purchaser be charged $10 * 50 + $9.90 * 50 = $995 (purchaser saves $5)
Would the purchaser be charged $1,000 and the price was increased from $9.90 to $10 (seller gains unexpected $5)
Would the exchange purchase the shares at $9.90 and quickly resell them for $10 (exchange benefit)
If #4 occurs, should the exchange still charge a fee on top of profits already made?
What is expected in this scenario?
To put this in better context, I'm comparing #4 to SEC Rule 605 to encourage open reporting and trade equality. Namely requiring market centers (Mt Gox) to disclose the extent to which they provide executions at prices better than the public quotes to investors using limit orders.