Interesting. What do you mean by negatively selected?
Negatively selected - i.e. you getting a desired outcome but the longer-term result will be "statistically bad". Let me offer you an analogy.
Imagine that you go to a club and a girl starts hitting on you. It's unusual, but you think of yourself as a proper stud and take in stride. You buy her drinks, she takes you home. You have wild sex and wake up the whole neighborhood. Sweet!
Next day you got back and get picked up by a different young lady. Again, drinks, snacks and you head to her place. However, instead of fornicating, you wake up in a ditch, with a headache and without your wallet. You write it off as bad luck and go to the same club the next day.
This time around, yet another girl but the same sad outcome as the second day - ditch, headache and missing wallet. You start realizing that "money for nothin' get chicks for free" actually comes a cost, i.e. sometimes you do get to that coveted third base but at cost of potentially losing your wallet 2/3rds of the time. By going to that club and responding to these chicks (i.e. desired short term outcome), you are getting negatively selected (getting robbed as a longer term result).
In case of PFOF, when you get filled (at NBBO or with improvement), you'd find that the price will frequently move against you right away. Whenever you miss a trade, you'll find that the price would have moved in your favor right after. Just like in the club, you are getting fucked, minus the condoms.
I have done meager post trade TCA, but what can I do as a child trying to stand on books to see over the horizon of my abusive stepfather's poker table? The fill is the fill. You're saying I should peg to stock and leg into complex orders?
Well, you should look at the price of your fill and compare it to the price some epsilon later. You'll find that you get picked off. Similarly, if you see yourself not getting filled at your limit even though it trades there or through elsewhere, see what the price would have been an epsilon later - that's opportunity cost. Do that many times and you'll find that despite it being free, you are indirectly paying for it.