The diamond is the same, but the promise of exact grading is missing. Since the VALUE is determined by a known shape, weight, color, clarity, and cut you are literally taking a blind shot in the dark by agreeing to just "Take someone''s word for it." Besides, the seller is certainly seen to have some bias in their recommendation.
For many decades the seller was just trusted to be honest. Some people had wonderful reltionships with trusted retailers in every field, not just jewelry. In recent years, doubt has entered the consumer market. Now, third party grading is considered a safer way to proceed, but you always can be impulsive or a risk taker. Its a personal choice.
There are still some jewelers whose word and knowledge is "take it to the bank" safe. Bur, how do you know unless you have a long standing, close and knowledgeable relationship. Otherwise, playing it safer is a better way.