Unless you're using the singular "they" in English... and we have records of respected authors using that as far back as 1395.
It only fell out of favour because "experts" kept insisting that we should use "he" to refer to people of indeterminate gender. ("he" as an indeterminate gender pronoun has been around for about the same amount of time.)
I think ça is only a demonstrative pronoun. When you say "that dress" you are using a demonstrative adjective (i.e., you're using "that" to describe the dress).
To say "that dress" you have to use a demonstrative adjective (ce, cet, cette, ces) so you'd say: "cette robe" since dress is feminine.
I believe saying "ça robe" is almost like saying "it dress" in English, it's just a bit off.
"That dress" would be "cette robe", not "ça robe". http://french.about.com/od/grammar/a/indefinite-demonstrative-pronoun.htm
I don't think you'll be able to distinguish ça from sa in hearing, but "sa," as an adjective, should generally be found preceding a noun (or perhaps an adjective followed by a noun), whereas "ça," as a pronoun, should generally be followed by a verb (or perhaps a reflexive pronoun followed by a verb) or come at the end of a clause.