In Tuck Everlasting, the unnamed man in the yellow suit serves as an antagonist. Though he seems friendly enough, it becomes obvious that he has nefarious intentions, seeking the Tuck's spring of everlasting life for his own gain. Once he realizes that Winnie has been taken back to the Tuck's home, he uses the knowledge of her location to strike a deal with her parents for the wooded area on which the spring is located.
With the land secured, he rides for the Tuck's farm in order to inform them of his acquisition. Before claiming the spring by force, he offers to allow the Tucks to join in with him on his business enterprise of selling the water from the spring.
This offer tells us a great deal about the man. For starters, he is primarily concerned with money, even more than the prospect of immortality. Furthermore, he is extraordinarily myopic, as he has considered none of the ramifications of selling immortality to anyone who has the money. Finally, he has no concept of something being sacred or protected.
It seems preposterous to assume for a second that the Tucks, who he knew to be hiding away from society to protect themselves, would even begin to consider this offer. The man in yellow likely does many "mental gymnastics" to make himself seem morally upright, so that he can cast the blame on the Tucks for refusing him.