In "Tuck Everlasting", what is the proposition the man in the yellow suit gives the Tucks?

The man in the yellow suit proposes that the Tucks show him the spring and help him sell it to wealthy people. He would do things to them in demonstrations to people to show the people that the Tucks are immortal. He essentially wants the Tucks to be "freaks...in a patent-medicine show" as Jesse describes it (Chapter 19). The Tucks find the idea appalling and refuse to help him, so he grabs Winnie and tells them that he will make Winnie drink the water and be his advertisement.