The Dragon's Whisker Salesman is the original owner of the Dragon's Whisker who handed the item over to Ranma Saotome after he had consumed some rice porridge which was created using a portion of the Whisker.
Shortly after acquiring his Jusenkyo curse, Ranma comes across a thief stealing some porridge belonging to the Salesman. After swiftly dispatching the thief, the starving Ranma consumes the porridge without a second thought. This worries the Salesman as the porridge was made using the Dragon's Whisker and would thus cause any male consumer's hair to grow uncontrollably. Luckily Ranma is currently in his female form, so is unaffected by the Whisker.
The Salesman then takes Ranma back to his restaurant where he explains about the effect Dragon's Whisker has on those who consume it. Once the Salesman finishes his explanation, however, Ranma reveals his true gender, leaving the Salesman with no choice but to give Ranma the rest of the Dragon's Whisker so that it's effects may be contained until they ultimately wear off.
By the time of the events of the series, the Salesman is found by the Niku-men who want the Whisker to cure their baldness. Since he no longer posses the Whisker, however, the Salesman informs the Niku-men to go to Japan so that they can find Ranma. The Salesman later meets with Shampoo who he teaches how to make the porridge using the Whisker, shortly before it's 4,000 year-old shelf life runs out.