I am looking for a site that allows high school students to sell
essays that they have previously written for a class to other students
who can buy the essays.  I have found many sites that allow college
students to do this, but none for high school students.  I need 1-3
sites that will allow someone to do this, and that specifically say that
they allow high school papers.  I am not looking for sites to buy
essays from, nor do I want sites that will accept essays in exchange
for free site membership.

I would avoid doing this if I were you - there are easier ways to make
money.  If you sell an essay, you are potentially assisting (or
conceivably could be acused of inciting) fraud.  If you have ever been
a student at the establishments where the assignments, for which you
intend to sell essays, were devised, then you risk being stripped of
qualifications and/or sued for breach of the implicit contract between
school and student.  It is also a breach of the Terms & Conditions or
some ISPs to use their services in this way, so you could get hassle
from that direction too.

Yes, but legally, don't I own the copyright to an essay?  I wrote it,
aren't I the legal owner of it?  In theory, another student could be
using it as research or the basis for a paper.  I would be interested
in understanding why I would not be legally allowed to sell a paper
which I myself have written.

It is true that the author of the essay would hold the copyright
unless he specifically transferred that right. It does not even need
to be registered or published. If, as in your example, a student used
a paper as a basis for his research, he would certainly have a lot of
explaining to do as any educator would question the "expert"
qualifications of another high school student as a cited reference.

I understand that.  I personally would never buy an essay from one of
those sites, not only is it cheating (which has very serious
consequences), but I feel I loose a lot from an educational
perspective if I just "purchased" an essay.  However I know many
students out there that do this, and I have no problem offering
already handed in and graded work to them, for a few spare bucks.  I
would assume that when I hand in an essay to be graded I am not
transfering that copyright?

Handing in a paper does not transfer copyright. In order to transfer
copyright a written contract is required between the parties involved.
There is a problem with handing over your paper to an agency which
terryjohnson alluded to earlier. Consider the following; your school's
administration would not be happy if they discovered that you handed
over your paper to another student at your school for their use on an
assignment. So, why would they allow you to do the same for a student
at another school? In other words, if you were discovered, you would
face serious consequences. As an example, at one academic institution,
the nature of the offence may be categorized as one or both of the
following:
Cheating
Unauthorized Possession or Disposition of Academic Materials
http://www.colorado.edu/policies/acadinteg.html

So, although it is technically legal, it may get you into alot of
trouble.

I see your point, its a good one.