Display List<String> in textBox (Winforms)

Once you return s.ToString(), the rest of that method stops running.
A method cannot return multiple things.

You probably want to write

someTextBox.Text = String.Join(Environment.NewLine, vegetables);

Use string.Join():

textbox1.Text = string.Join(" ", vegetables);

The answer was closer and easier than you thought :)

Note: as mentioned in the comments, this particular function has several overloads added in v4.0 of the .Net framework. If you want to see what is available for earlier versions use the Other Versions dropdown just under the title on the MSDN doco page.

You need to concatenate the strings somehow, like

public string displayMembers(List<String> vegetables)
{
    return string.Join(", ", vegetables.ToArray());
}

or

public string displayMembers(List<String> vegetables)
{
    return string.Join(Environment.NewLine, vegetables.ToArray());
}