Currently the 820 is available in 13,16, 18, 19.5 and 21
Translated, that means you have a smaller and a medium sized Trek 820. You can go to
http://www.trekbikes.com/us/en/bikes...dtail/820/820/
to see specs on a new 820.
If it was me, I would drive to see them and adjust the price down if it looks like they need tires, etc. If you have a bike mechanic friend, take him along and buy him lunch.
Best of luck and Merry Christmas!