What do you mean by 16 bit?

If you mean pixel art then just say pixel art. Saying you are doing a 16 bit game doesn't really have any meaning unless you are developing a game for a 16 bit platform.

I have an assignment to come up with a game idea, and pitch it. My idea is to create a
full story 16 bit RPG game, no big deal. The concept, though, is to use the game as an
on site advertisement for a small to medium sized retail website.
So my question is, what would be the best way to do a quick elevator pitch of this. I
was thinking that I could do an audio pitch, a short video pitch or a magazine style ad
or pamphlet style ad.
Any opinions, and please include reason for your choice.

So to satisfy a college/university? assignment, you plan to actually/hypothetically?
create a "full" RPG, then repurpose it as an advergame for, and residing on, a retail
site.

That idea doesn't make much sense. Advergames by nature need to be quick/short games to
play, not lengthy tomes. Also consider the audience - who are the people who shop on
that retail site? What percentage of those shoppers do you think really want to spend
hours playing an RPG? If you answer anything other than "very low," you're not being
honest with yourself.

Secondly, your college/university? assignment is to create a pitch plan. Consider who
you have to pitch to - the online retailer. You have to convince the retailer that lots
of their shoppers are going to spend hours playing your RPG, and that those hours somehow
will convert to retail revenue. Tough sell.

Thirdly, we really don't like helping students with their homework here. We prefer
students do real research, by looking for existing information.