Charges are flat rate. $50 on $50 is 100%. $50 on $5000 is 1%. $50 on $500k is 0.01%. I could go on but I'm guessing you get the point.
Right. So now you bought 100 instead. That's a profit of 500-50=450. I'm not seeing the issue? The more investors had put in, the higher the profit would have been. Just like any other trading system in the world. It's nothing new/complicated. But that also doesn't mean it isn't risky.
Except for the slippage. You can't scale arbitrage all the way up. Because buying on Site A will make the price go up there, and Selling on site B will make the price go down there, meaning you will make less and less profit every time you make the trade, which is why we gave you the correct answer that should be given to everyone claiming to have an arbitrage opportunity, "why aren't you doing it before it goes away?"
Because not everyone has the cash on hand at that moment? Why does Wal-Mart take out loans instead of just spending cash to build out more locations? Same reason.