It seems like you don't like bitfinex 
Why don't you change your signature.
And also, I was thinking of how day trading or something of this nature would work. Lets say a btc is bought for 500 and then sold at 510 a few min later. Then a btc is bought in at 490. I am just thinking that the person that gained from this quick trading made a profit (at a cost of whoever bought it). I feel like the exchanges might have to eat those costs (so maybe they just try to avoid eating all the extra costs).
This is just a thought and I do not know about how bitfinex works (seem complex). But everyone gotta do whats best for them in the end.
Why don't you change your signature.
And also, I was thinking of how day trading or something of this nature would work. Lets say a btc is bought for 500 and then sold at 510 a few min later. Then a btc is bought in at 490. I am just thinking that the person that gained from this quick trading made a profit (at a cost of whoever bought it). I feel like the exchanges might have to eat those costs (so maybe they just try to avoid eating all the extra costs).
This is just a thought and I do not know about how bitfinex works (seem complex). But everyone gotta do whats best for them in the end.
I don't like Bitfinex. Inviting people to trade Bitfinex where they can swim with sharks and wrestle with vipers is hardly a ringing endorsement of the place, I wouldn't have thought.
But there is another stone wall example of crooked Bitfinex in action. There was around 600 BTC in between $599 and $605, with all the orders of course at different price levels. Yet here is the trading for the break above 600 btc:
02:00:58 602.29 0.1
02:00:56 600.3 1.00626
02:00:55 608.13 319.168
02:00:24 600 255.374
02:00:17 599.91 0.01
In green, is the Ask wall which my stop loss order was sitting behind. There was as any reasonably minded person could imagine, a whole array of other double figure and single figure and indeed decimal bids in between 600 and 608. Why therefore was the next 319.168 of bids all sold at 608.13, my paltry 6 BTC amongst them when there was a whole raft of bids in between $600 and $608!?
BITFINEX CAUGHT RED HANDED STEALING!
If this is true, how come when I watched the buys go by live, it didnt jump to $608? Instead it ate thru $600 and then went thru shallow waters to $608?
The only oddity I saw was that shortly after breaking $600 and moving to ~603, someone put a 100.7BTC ask at $600 again, that got eaten really quick. This was seconds after $600 went down, and the 100B wall was eaten in seconds after being placed.