Final answer:
The type of mistake involved in the scenario is a mutual mistake, where both parties believed the golf clubs were used by a famous golfer, but later discovered it was incorrect.
Step-by-step explanation:
The type of mistake in this scenario is a mutual mistake.
In this case, both Ned and Ira believed that the golf clubs were used by a famous golfer, which influenced their decision to negotiate and sell them at a higher price. However, they later discovered that their belief was incorrect, significantly reducing the value of the golf clubs.
This mutual mistake occurred because both parties had the same mistaken understanding about the golf clubs' provenance, leading to a false assumption and a less favorable outcome for Ira.