"Retrading" Vs. What Ken McElroy Indicates

[Jason Merchey]

I was reading THe ABCs of REI by Ken McE and came across this: 

"[In doing due diligence], I take comfort in knowing that anything I find that would cost me money to fix is a final negotiation point before the sale closes." (p.113)

Retrading is what I think he is talking about here, and other sources have said stuff like if you try to retrade, don't expect to last very long in this industry. Brokers don't like it. 

So what am I missing?