Sammy the Salesman is a minor character in Harvest Moon 64.
He is a shady dealer of goods who often tries to swindle the player out of good money. Every so often he will show up at your door trying to sell you "improved" products for inflated prices. He is very persistent and if you don't want to buy it you will have to refuse him several times. All of these sales pitches are in fact scams.
A Power Berry can be legitimately acquired from him, however, for the price of 1000G, when he visits during the Flower Festival. If you refuse to buy it, he will slip and make the comment that "it's real this time."