Evaluating a "blank"

A few months ago I found a trash pile at work that appeared to have a couple of large Newtonian objectives on top (I work in a physics department, so this is not overly strange.) After doing the initial double-take I found a ruler and went back for a closer look. They were 13" and 9", and both were mounted in adjustable cells. However, the cells didn't look like anything that was supposed to go at the end of a tube. I wasn't completely sure that these were being discarded, so I asked a friend in the Astrophysics group to inquire for me. Eventually, I got the word that these were indeed trash, but by this time the 13" mirror had disappeared. Since I already have a 10" scope I wasn't thrilled with taking the 9", but my friend convinced me it could be used to fund my eyepiece habit, so I grabbed the mirror and stuck it in the corner of my office

...Fast forward to present day...

I am now getting ready to move across the country, so figured it is time to get rid of all my junk. Since I don't have a Foucault tester handy, I figured I would just sell this glass as a blank and let the buyer have a pleasant surprise if the figure happens to be good. Still, to be thorough, I wanted to post the focal ratio so they buyer would know what kind of curve they were getting. Here is the punchline...this thing is completely flat! I think I might have an optical flat on my hands. IIRC, flats are worth a lot more than curved mirrors, correct? By how much? More importantly, how can I confirm that it is indeed an optical flat and check the quality? I don't want to sell this thing and screw the buyer over if it happens to have a crappy surface. Most of the reflective coating is missing (about 80%), but otherwise it looks like the surface is in pretty good shape. That protective layer of dust must have worked. Any suggestions for how to get rid of this thing?

Without some test or certification the mirror
is basically a "blank" in terms of pricing.

For testing you will need to remove the
remaining coating. This could be done with
some PC Board Etchant solution from Radio Shack.
Use gloves and safety googles. Put the mirror
in a plastic tub. Do this outdoors (fumes).
Pour some of the solution on the mirror, and
use a soft paint brush to work it around the surface.
Agitate now and then with paint brush. Probably
it will take 15 to 20 minutes. When
coating is gone rinse thoroughly with water
and then distilled water.

Testing: Use your 10" scope and Polaris.
Setup your 10" scope outside aiming generally north.
Put the flat on the ground north of the scope --
try to support it evenly, like on some soft foam
rubber. Point the scope at the flat.
Arrange so that you can view the reflection
of Polaris off the flat and into your 10"
scope. You will want the scope drive off.
You will need to do a little work to get the
flat positioned in the center of the 10"
scope's aperture by slidng the flat around
sideways. Do a star test on Polaris at about
500x. The most likely error you might see will be
astigmatism if the flat is not flat, but instead
is slightly concave or convex. You can judge
the accuracy of the flat in the same way as a
telescope via the star test. With a careful eye
you can see errors down to maybe 1/10 wavelength
on the wavefront. (Star Testing is a large subject
itself which I will not dive into.)