As my father says. "A good trade is when both the buyer and seller are unhappy".
But a great trade is when the both are happy
I suppose the logic is
"Rats, if he agreed to pay 100$, he probably would have paid 105$; I should have bargained harder."
"Rats, if he agreed to sell for 100$, he probably would have sold for 95$; I should have bargained harder."