I clearly remember finding an item online a few years ago which helped
in the trimming of "back of the neck" hair.  Basically, imagine a
sunglasses strap like this:
http://www.4imprint.com/EXEC/DETAIL/~SKU007488/~CA7488.htm
However, the one that I saw was more firm and malleable, and
specifically marketed for helping with trimming the back of the neck
hair.  I think it
might have come with some pseudo-glasses to keep it on the neck while using it. 
Anyway, it looked like it could be easily shaped to match the hairline
of the back of the neck, and make shaving the hair below the hairline
much easier, since you just shave until you bump into the device.  I
have done many searches on eBay and google since then and have been
unable to find the device online.  I would like a link to
purchase the device, whether through a private classified ad/auction
or through an online retail site.  Thanks.

I think this is what you had in mind:
http://www.haircut.com/hairline.html

from the site:
"If you do haircuts that require shaving the neck on your own head,
you know the challenge: the ends of the hair should form a straight
line, not too high, not too low, not crooked, that curves just right
toward the ears. It requires two mirrors and much coordination. The
HairLine does that for you: just put it on like a reversed
sun-glasses, affix the strap(optional) and use the razor touching the
HairLine as a template. No need for mirror, so you can use it even in
the shower."

You can buy it directly from that site.
You can also buy it from:
http://shop.store.yahoo.com/888haircut/hairline1.html

Best regards,
hagan-ga

Excellent, thanks!