If the 800 is in good cosmetic and mechanical condition, $80 is okay. The frames are bulletproof. If the consumables don't need to be replaced; cables, housing, brake pads, tires, grips, go for it. On an 800, the most vulnerable parts are the twist grip shifters, cheap and can fall apart. If they are bad and don't shift precisely, knock $25 off your offer and replace them with thumb shifters.

If you can spend a few more bucks, look for a Trek 850 or 930. STX and Deore shifters and derailleurs are more robust. Specialized Rockhopper and Stumpjumpers are very nice vintage MTBs too.

Looking on the Bellingham Craigslist, there are some of each, but I don't know what size you would need and if that's close enough to you.