"if someone buys an 88, then the market drops to 80 and they buy another share at 80 and want to flip at 81. Their only choice is to sell the 88 at a loss. That trader might want to sell the profitable share first."
. . . Maybe I'm misunderstanding this, but why would you want to sell the profitable shares first? You would have to pay the 10% fee on the profit. If the price goes down afterwards, below 81 and you then sell your shares you bought for 81, you wouldn't have to pay the fee.
. . . I would like to always sell the less profitable shares first, to save on that ugly 10% fee (the 5% fee to play in the markets is fine, but the 10% fee on profits distorts behavior).